Organic jewelry

Organic jewelry
Varieties
1.    Amber
2.    Jet
3.    Ivory
4.    Coral
5.    Tortoise Shell
6.    Shell
7.    Pearl
Amber
Amber (or, technically, resinite) is fossilized tree resin (not sap), which has been appreciated for its color and natural organic beauty since Neolithic times.Amber is used as an ingredient in perfumes, as a healing agent in folk medicine, and as jewelry. There are five classes of amber, defined on the basis of their chemical constituents. Because it originates as a soft, sticky tree resin, amber sometimes contains animal and plant material as inclusions.

History and etymology
The English word amber derives from the Arabic 'anbar, via Medieval Latin ambar and Old French ambre. The word originally referred to a precious oil derived from the Sperm whale (now called ambergris). The sense was extended to fossil resin circa 1400, and this became the main sense as the use of ambergris waned.The two substances were confused because they both were found washed up on beaches. Ambergris is lighter than water and floats; whereas amber is lighter than stone, but not light enough to float.The word "ambar" was brought to Europe by the Crusaders. In French "ambre gris" was then distinguished from "ambre jaune": ambre gris was ambergris; ambre jaune was the fossil resin we now call amber.
Amber is discussed by Theophrastus, possibly the first historical mention of the material, in the 4th century BC. The Greek name for amber was ηλεκτρον (electron) and was connected to the Sun God, one of whose titles was Elector or the Awakener.The modern terms "electricity" and "electron" derive from the Greek word for amber and come from William Gilbert's research showing that amber could attract other substances.The word "electron" was coined in 1891 by the Irish physicist George Stoney whilst analyzing elementary charges for the first time.
The presence of insects in amber was noticed by Pliny the Elder in his Naturalis Historia, and led him to theorise correctly that, at some point, amber had to be in a liquid state to cover the bodies of insects. Hence he gave it the expressive name of succinum or gum-stone, a name that is still in use today to describe succinic acid as well as succinite, a term given to a particular type of amber by James Dwight Dana (see below under Baltic Amber).
Heating amber will soften it and eventually it will burn, which is why in Germanic languages the word for amber is a literal translation of burn-Stone (In German it is Bernstein, in Dutch it is barnsteen etc.). Heated above 200°C, amber suffers decomposition, yielding an "oil of amber", and leaving a black residue which is known as "amber colophony", or "amber pitch"; when dissolved in oil of turpentine or in linseed oil this forms "amber varnish" or "amber lac".
Amber from the Baltic Sea has been extensively traded since antiquity and in the main land, from where amber was traded 2000 years ago, the natives called it glaes (referring to its see-through similarity to glass).
The Baltic Lithuanian term for amber is Gintaras and Latvian Dzintars. They and the Slavic jantar are thought to originate from Phoenician jainitar (sea-resin). However, while most Slavic languages, such as Russian and Czech, retain the old Slavic word, in the Polish language, despite still being correct, jantar is used very rarely (even considered archaic) and was replaced by the word bursztyn deriving from the German analogue.

Composition and formation

Amber is heterogeneous in composition, but consists of several resinous bodies more or less soluble in alcohol, ether and chloroform, associated with an insoluble bituminous substance. Amber is a macromolecule by free radical polymerization of several precursors in the labdane family, e.g. communic acid, cummunol, and biformene.These labdanes are diterpenes (C20H32) and trienes, equipping the organic skeleton with three alkene groups for polymerization. As amber matures over the years, more polymerization takes place as well as isomerization reactions, crosslinking and cyclization.
The average composition of amber leads to the general formula C10H16O.

Formation

Molecular polymerization, resulting from high pressures and temperatures produced by overlying sediment, transforms the resin first into copal. Sustained heat and pressure drives off terpenes and results in the formation of amber.

Botanical origin

Fossil resins from Europe fall into two categories, the famous Baltic ambers and another that resembles the Agathis group. Fossil resins from the Americas and Africa are closely related to the modern genus Hymenaea.

Inclusions

The abnormal development of resin has been called succinosis. Impurities are quite often present, especially when the resin dropped on to the ground, so that the material may be useless except for varnish-making, whence the impure amber is called firniss. Enclosures of pyrites may give a bluish color to amber. The so-called black amber is only a kind of jet. Bony amber owes its cloudy opacity to minute bubbles in the interior of the resin.
In darkly clouded and even opaque amber, inclusions can be imaged using high-energy, high-contrast, high-resolution x-rays.

Extraction and processing

Distribution and mining

Amber is globally distributed, mainly in rocks of Cretaceous age or younger. Historically, the coast around Königsberg in Prussia was the world's leading source of amber; about 90% of the world's extractable amber is still located in the Kaliningrad Oblast of Russia on the Baltic Sea.Pieces of amber torn from the seafloor are cast up by the waves, and collected by hand, dredging or diving. Elsewhere, amber is mined, both in open works and underground galleries. The nodules from the blue earth have to be freed from matrix and divested of their opaque crust, which can be done in revolving barrels containing sand and water. Erosion removes this crust from sea-worn amber.
Dominican amber, especially Dominican blue amber, is mined through bell pitting, which is dangerous due to the risk of the tunnel collapse.

Treatment

The Vienna amber factories, which use pale amber to manufacture pipes and other smoking tools, turn it on a lathe and polish it with whitening and water or with rotten stone and oil. The final lustre is given by friction with flannel.
When gradually heated in an oil-bath, amber becomes soft and flexible. Two pieces of amber may be united by smearing the surfaces with linseed oil, heating them, and then pressing them together while hot. Cloudy amber may be clarified in an oil-bath, as the oil fills the numerous pores to which the turbidity is due. Small fragments, formerly thrown away or used only for varnish, are now used on a large scale in the formation of "amberoid" or "pressed amber". The pieces are carefully heated with exclusion of air and then compressed into a uniform mass by intense hydraulic pressure; the softened amber being forced through holes in a metal plate. The product is extensively used for the production of cheap jewelry and articles for smoking. This pressed amber yields brilliant interference colors in polarized light. Amber has often been imitated by other resins like copal and kauri, as well as by celluloid and even glass. Baltic amber is sometimes colored artificially, but also called "true amber".

Counterfeit amber

Often amber (particularly with insect inclusions) is counterfeited using a plastic resin. A simple test consists of touching the object with a heated pin and determining if the resultant odor is of wood resin. If not, the object is counterfeit, although a positive test may not be conclusive owing to a thin coat of real resin. Generally counterfeits will have a too-perfect pose and position of the trapped insect.

Appearance

Amber occurs in a range of different colors. As well as the usual yellow-orange-brown that is associated with the color "amber", amber itself can range from a whitish color through a pale lemon yellow, to brown and almost black. Other more uncommon colors include red amber (sometimes known as "cherry amber"), green amber, and even blue amber, which is rare and highly sought after.
Much of the most highly-prized amber is transparent, in contrast to the very common cloudy amber and opaque amber. Opaque amber contains numerous minute bubbles. This kind of amber is known as "bony amber".
Although all Dominican amber is fluorescent, the rarest Dominican amber is blue amber. It turns blue in natural sunlight and any other partially or wholly ultraviolet light source. In long-wave UV light it has a very strong reflection, almost white. Only about 100 kg is found per year, which makes it valuable and expensive.
Sometimes amber retains the form of drops and stalactites, just as it exuded from the ducts and receptacles of the injured trees. It is thought that, in addition to exuding onto the surface of the tree, amber resin also originally flowed into hollow cavities or cracks within trees, thereby leading to the development of large lumps of amber of irregular form.

Classification

Amber can be classified into several forms. Most fundamentally, there are two types of plant resin with the potential for fossilization. Terpenoids, produced by conifers and angiosperms, consist of ring structures formed of isoprene (C5H8) units.Phenolic resins are today only produced by angiosperms, and tend to serve functional uses. The extinct medullosans produced a third type of resin, which is often found as amber within their veins.The composition of resins is highly variable; each species produces a unique blend of chemicals which can be identified by the use of pyrolysis–gas chromatography–mass spectroscopy.The overall chemical and structural composition is used to divide ambers into five classes.There is also a separate classifications of amber gemstones, according to the way of production.
Class I:
This class is by far the most abundant. It comprises labdatriene carboxylic acids such as communic or ozic acids.It is further split into three sub-classes. Classes Ia and Ib utilise regular labdanoid diterpenes (e.g. communic acid, communol, biformenes), whilst Ic uses enantio labdanoids (ozic acid, ozol, enantio biformenes).
Ia:
Includes Succinite (= 'normal' Baltic amber) and Glessite.Have a communic acid base. They also include much succinic acid.
Baltic amber yields on dry distillation succinic acid, the proportion varying from about 3% to 8%, and being greatest in the pale opaque or bony varieties. The aromatic and irritating fumes emitted by burning amber are mainly due to this acid. Baltic amber is distinguished by its yield of succinic acid, hence the name succinite. Succinite has a hardness between 2 and 3, which is rather greater than that of many other fossil resins. Its specific gravity varies from 1.05 to 1.10. It can be distinguished from other ambers via IR spectroscopy due to a specific carbonyl absorption peak. IR spectroscopy can detect the relative age of an amber sample.Succinic acid may not be an original component of amber, but rather a degradation product of abietic acid.
Ib:
Like class Ia ambers, these are based on communic acid; however, they lack succinic acid.
Ic:
This class is mainly based on enantio-labdatrienonic acids, such as ozic and zanzibaric acids.Its most familiar representative is Dominican amber.
Dominican amber differentiates itself from Baltic amber by being mostly transparent and often containing a higher number of fossil inclusions. This has enabled the detailed reconstruction of the ecosystem of a long-vanished tropical forest.Resin from the extinct species Hymenaea protera is the source of Dominican amber and probably of most amber found in the tropics. It is not "succinite" but "retinite".
Class II:
These ambers are formed from resins with a sesquiterpenoid base, such as cadinene.
Class III:
These ambers are polystyrenes.
Class IV:
This class is something of a wastebasket; its ambers are not polymerized, but mainly consist of cedarane-based sesquiterpenoids.
Class V:
Class V resins are considered to be produced by a pine or pine relative. They comprise a mixture of diterpinoid resins and n-alkyl compounds. Their type mineral is Highgate Copalite.

Classification of Baltic amber (succinite) gemstones by the International Amber Association

Natural Baltic amber – gemstone which has undergone mechanical treatment only (for instance: grinding, cutting, turning or polishing) without any change to its natural properties
Modified Baltic amber – gemstone subjected only to thermal or high-pressure treatment, which changed its physical properties, including the degree of transparency and color, or shaped under similar conditions out of one nugget, previously cut to the required size.
Reconstructed (pressed) Baltic amber – gemstone made of Baltic amber pieces pressed in high temperature and under high pressure without additional components.
Bonded Baltic amber – gemstone consisting of two or more parts of natural, modified or reconstructed Baltic amber bonded together with the use of the smallest possible amount of a colorless binding agent necessary to join the pieces.

Geological record

The oldest amber recovered dates to the Upper Carboniferous period (320 million years ago).Its chemical composition makes it difficult to match the amber to its producers - it is most similar to the resins produced by flowering plants, which did not evolve until the Jurassic, around 180 million years ago. Amber becomes abundant soon afterwards, in the Early Cretaceous, 150 million years ago,when it is found in association with insects.
Baltic amber or succinite (historically documented as Prussian amber) is found as irregular nodules in marine glauconitic sand, known as blue earth, occurring in the Lower Oligocene strata of Samland in Prussia (Latin: Sambia), in historical sources also referred to as Glaesaria. After 1945 this territory around Königsberg was turned into Kaliningrad Oblast, Russia, where it is now systematically mined.It appears, however, to have been partly derived from yet earlier Tertiary deposits (Eocene); and it occurs also as a derivative phase in later formations, such as glacial drift. Relics of an abundant flora occur as inclusions trapped within the amber while the resin was yet fresh, suggesting relations with the flora of Eastern Asia and the southern part of North America. Heinrich Göppert named the common amber-yielding pine of the Baltic forests Pinites succiniter, but as the wood does not seem to differ from that of the existing genus it has been also called Pinus succinifera. It is improbable, however, that the production of amber was limited to a single species; and indeed a large number of conifers belonging to different genera are represented in the amber-flora.

Paleontological significance

Amber is a unique preservational mode, preserving otherwise unfossilizable parts of organisms; as such it is helpful in the reconstruction of ecosystems and organisms.
The chemical composition of the resin is, unfortunately, of limited utility in reconstructing the phylogenetic affinity of the resin producer.
Amber sometimes contains animals or plant matter that became caught in the resin as it was secreted. Insects, spiders and their webs, annelids, frogs,crustaceans, bacteria and amoebae,marine microfossils,wood, flowers and fruit, hair, feathers and other small organisms have been recovered in ambers dating to 130 million years ago.In most cases the original organic material has decayed, leaving only a cavity, and sometimes remnants of resistant materials such as chitin.

Use

Amber has been used since antiquity in the manufacture of jewelry and ornaments, and also in folk medicine. Amber also forms the flavoring for akvavit liquor. Amber has been used as an ingredient in perfumes.

Jewelry

Amber has been used since the stone age, from 13,000 years ago.Amber ornaments have been found in Mycenaean tombs and elsewhere across Europe. To this day it is used in the manufacture of smoking and glassblowing mouthpieces. Amber's place in culture and tradition lends it a tourism value; Palanga Amber Museum is dedicated to the mineral.

Historic medicinal uses

Amber has long been used in folk medicine for its purported healing properties. Amber and extracts were used from the time of Hippocrates in ancient Greece for a wide variety of treatments through the Middle Ages and up until the early twentieth century.Prior to World War II amber was widely used in Germany for teething problems. Amber's healing properties are sometimes attributed to succinic acid which occurs as an oxidation product on the surface of amber.

Scent of amber and amber perfumery

In ancient China it was customary to burn amber during large festivities. If amber is heated under the right conditions, oil of amber is produced, and in past times this was combined carefully with nitric acid to create "artifical musk" - a resin with a peculiar musky odor.Although when burned, amber does give off a characteristic "pinewood" fragrance, modern products, such as perfume, do not normally use actual amber. This is due to the fact that fossilized amber produces very little scent. In perfumery, scents referred to as “amber” are often created and patented to emulate the opulent golden warmth of the fossil.The modern name for amber is thought to come from the Arabic word, ambar, meaning ambergris.Ambergris is the waxy aromatic substance created in the intestines of sperm whales and was used in making perfumes both in ancient times as well as modern. The scent of amber was originally derived from emulating the scent of ambergris and/or labdanum but due to the endangered status of the sperm whale the scent of amber is now largely derived from labdanum.The term “amber” is loosely used to describe a scent that is warm, musky, rich and honey-like, and also somewhat oriental and earthy. It can be synthetically created or derived from natural resins. When derived from natural resins it is most often created out of labdanum.Benzoin is usually part of the recipe. Vanilla and cloves are sometimes used to enhance the aroma.
"Amber" perfumes may be created using combinations of labdanum, benzoin resin, copal (itself a type of tree resin used in incense manufacture), vanilla, Dammara resin and/or synthetic materials.
Jet
Jet is a geological material and is considered to be a minor gemstone. Jet is not considered a true mineral, but rather a mineraloid as it has an organic origin, being derived from decaying wood under extreme pressure.
The English noun "jet" derives from the French word for the same material: jaiet.Jet is either black or dark brown, but may contain pyrite inclusions, which are of brassy colour and metallic lustre. The adjective jet-black is better known perhaps than the substance from which the descriptive phrase derives.

Origin

Jet is a product of high pressure decomposition of wood from millions of years ago, commonly the wood of trees of the family Araucariaceae. Jet is found in two forms, hard and soft. Hard jet is the result of the carbon compression and salt water; soft jet is the result of the carbon compression and fresh water.
The jet found at Whitby, England is of early Jurassic (Toarcian) age, approximately 182 million years old.
Jet is easily polished and is used in manufacturing jewellery, according to the Whitby Museum, dating from 10,000 BC in parts of contemporary Germany. The oldest jet jewellery was found in Asturias, Spain, dating from 17,000 BC.

History

Jet as a gemstone was fashionable during the reign of Queen Victoria, during which the Queen wore Whitby jet as part of her mourning dress. Jet was associated with mourning jewellery in the 19th century because of its sombre colour and modest appearance, and it has been traditionally fashioned into rosaries for monks. In the United States, long necklaces of jet beads were very popular during the 1920s, or Roaring Twenties, when women and young flappers would wear multiple strands of jet beads stretching from the neckline to the waistline. In these necklaces, the jet was strung using heavy cotton thread; small knots were made on either side of each bead to keep the beads spaced evenly, much in the same way that fine pearl necklaces are made. Jet has also been known as black amber, as it may induce an electric charge like that of amber when rubbed.

Properties

Jet is very easy to carve but it is difficult to create detail without breaking so it takes some time for learning and executing more elaborate carving.
Jet has a Mohs hardness ranging between 2.5 to 4 and a specific gravity of 1.30 to 1.34. The refractive index of jet is approximately 1.66. The touch of a red-hot needle should cause jet to emit an odor similar to coal.

Authenticating jet

Although now much less popular than in the past, authentic jet jewels are valued by collectors.
Unlike black glass, which is cool to the touch, jet is not cool, due to its lesser thermal conductivity.
Anthracite (hard coal) and vulcanite (hardened rubber) are superficially similar to fine jet, and have been used to imitate it. These imitations are not always easy to distinguish from the real thing.
The structure of jet (which is remarkably like the wood that it is derived from) can be seen under 120x or greater magnification.
Ivory
Ivory is a term for dentine, which constitutes the bulk of the teeth and tusks of animals, when used as a material for art or manufacturing. Ivory is little used today, but has been important since ancient times for making a range of items, from ivory carvings to false teeth, fans, piano keys and billiard balls. Elephant ivory has been the most important source, but ivory from many species including the hippopotamus, walrus, mammoth and narwhal has been used. The word ultimately derives from the Ancient Egyptian âb, âbu "elephant", through the Latin ebor- or ebur.
The use and trade of elephant ivory has become controversial as its harvesting, which often involves killing elephants solely for their tusks, has contributed to declining populations in some areas. Many organizations, including the United Nations under CITES, have banned the ivory trade, and nearly all governments prohibit harvesting. Poaching and smuggling in violation of such bans is a problem in some places.
Ivory has availed itself to many ornamental and practical uses. Prior to the introduction of plastics, it was used for billiard balls, piano keys, Scottish bagpipes, buttons and a wide range of ornamental items. Synthetic substitutes for ivory have been developed. Plastics have been viewed by piano purists as an inferior ivory substitute on piano keys, although other recently developed materials more closely resemble the feel of real ivory.
The chemical structure of the teeth and tusks of mammals is the same regardless of the species of origin. The trade in certain teeth and tusks other than elephant is well established and widespread, therefore "ivory" can correctly be used to describe any mammalian teeth or tusks of commercial interest which is large enough to be carved or scrimshawed (Crocodile teeth are also used).

Uses

Both the Greek and Roman civilizations practised ivory carving to make large quantities of high value works of art, precious religious objects, and decorative boxes for costly objects. Ivory was often used to form the white of the eyes of statues.
The Syrian and North African flacid elephant populations were reduced to extinction, probably due to the demand for ivory in the Classical world.
The Chinese have long valued ivory for both art and utilitarian objects. Early reference to the Chinese export of ivory is recorded after the Chinese explorer Zhang Qian ventured to the west to form alliances to enable for the eventual free movement of Chinese goods to the west; as early as the first century BC, ivory was moved along the Northern Silk Road for consumption by western nations.Southeast Asian kingdoms included tusks of the Indian elephant in their annual tribute caravans to China. Chinese craftsmen carved ivory to make everything from images of deities to the pipe-stems and end-pieces of opium pipes.
The Indianized Buddhist cultures of Southeast Asia, including Myanmar (Burma), Thailand, Laos and Cambodia traditionally harvested ivory from their domesticated elephants. Ivory was prized for containers due to its ability to keep an airtight seal. Ivory was also commonly carved into elaborate seals utilized by officials to "sign" documents and decrees by stamping them with their unique official seal.
In Southeast Asian countries where Muslim Malay peoples live, such as Malaysia, Indonesia and the Philippines, ivory was the material of choice for making the handles of magical kris daggers. In the Philippines, ivory was also used to craft the faces and hands of Catholic icons and images of saints.
Tooth and tusk ivory can be carved into a vast variety of shapes and objects. A small example of modern carved ivory objects are small statuary such as okimono, netsukes, jewelry, flatware handles, furniture inlays, and piano keys. Additionally, warthog tusks, and teeth from sperm whales, orcas and hippos can also be scrimshawed or superficially carved, thus retaining their morphologically recognizable shapes.

Consumption before plastics

Before plastics were invented, ivory was important for cutlery handles, musical instruments, billiard balls, and many other items. It is estimated that consumption in Great Britain alone in 1831 amounted to the deaths of nearly 4,000 elephants. Ivory can be taken from dead animals — Russians dug up tusks from extinct mammoths — however most ivory came from elephants who were killed for their tusks. Other animals which are now endangered were also preyed upon, for example, hippos, which have very hard white ivory prized for making artificial teeth.

Availability

Owing to the rapid decline in the populations of the animals that produce it, the importation and sale of ivory in many countries is banned or severely restricted. Much of the decline in population is due to poachers during and before the 1980s. Since the United Nations placed a nine-year moratorium on ivory in 1989, there have been ups and downs in elephant populations as bans have been lifted for poorer nations with large elephant populations. Many African countries—including Zimbabwe, Zambia, Namibia, and Botswana—claim that the ivory trade is necessary both to stimulate their economies and to cull large populations of elephants, which are allegedly harming the environment.
In 2007 eBay, under pressure from the International Fund for Animal Welfare, banned all international sales of elephant-ivory products. The decision came after several mass slaughters of African elephants, most notably the 2006 Zakouma elephant slaughter in Chad. The IFAW found that up to 90% of the elephant-ivory transactions on Ebay violated their own wildlife policies and could potentially be illegal. In October 2008, eBay expanded the ban, disallowing any sales of ivory on eBay.
In 2002 the United Nations partially lifted the ban on ivory trade, allowing a few countries to export certain amounts of ivory. Six years later, the UN-backed Convention on International Trade in Endangered Species granted China and Japan permission to import elephant ivory from African government stockpiles in a one-time auction. Approximately 44 tons of ivory from Botswana, 9 tons from Namibia, 51 tons from South Africa, and 4 tons from Zimbabwe were sold.
Kenya emerged as the main critic of these one-off sales. The country saw its elephant populations plummet in the decade preceding the 1989 ban, and claims that legalizing ivory trade anywhere in Africa will endanger elephants everywhere in Africa as poachers would attempt to launder their illegal ivory with legal stockpiles. In 2010, Kenya and 22 allies proposed a 20-year moratorium in ivory trade at a special session of the Doha Development Round.
Recently, there has been a steady increase in the ivory trade due to a recent increase in the population of the South African elephant population. The factors that have led to this recent boom in the animals population growth is currently unknown but conservationists believe that a reduction in poaching and the implementation of stricter bans on ivory trading are all contributing factors to this unprecedented phenomenon.

 Alternative sources

Trade in the ivory from the tusks of dead mammoths has occurred for 300 years and continues to be legal. Mammoth ivory is used today to make handcrafted knives and similar implements. Mammoth ivory is rare and costly, because mammoths have been extinct for millennia and scientists loathe to sell museum-worthy specimens in pieces, but this trade does not threaten any living species. However, the uncontrolled extraction of mammoth remains from the melting Siberian permafrost is a problem.
A species of hard nut is gaining popularity as a replacement for ivory, although its size limits its usability. It is sometimes called vegetable ivory, or tagua, and is the seed endosperm of the ivory nut palm commonly found in coastal rainforests of Ecuador, Peru and Colombia.
Coral
Corals are marine organisms in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans, which secrete calcium carbonate to form a hard skeleton.
A coral "head," which appears to be a single organism, is a colony of myriad genetically identical polyps. Each polyp is typically only a few millimeters in diameter. Over many generations the colony secretes a skeleton that is characteristic of the species. Individual heads grow by asexual reproduction of individual polyps. Corals also breed sexually by spawning. Polyps of the same species release gametes simultaneously over a period of one to several nights around a full moon.
Although corals can catch small fish and animals such as plankton using stinging cells on their tentacles, most corals obtain the majority of their energy and nutrients from photosynthetic unicellular algae called zooxanthellae. Such corals require sunlight and grow in clear, shallow water, typically at depths shallower than 60 metres (200 ft). Corals can be major contributors to the physical structure of the coral reefs that develop in tropical and subtropical waters, such as the enormous Great Barrier Reef off the coast of Queensland, Australia. Other corals do not have associated algae and can live in much deeper water, with the cold-water genus Lophelia surviving as deep as 3,000 metres (9,800 ft).Examples live on the Darwin Mounds located north-west of Cape Wrath, Scotland. Corals have also been found off the coast of the U.S. in Washington state and the Aleutian Islands in Alaska.

Taxonomy

Corals divide into two subclasses, depending on the number of tentacles or lines of symmetry, and a series of orders corresponding to their exoskeleton, nematocyst type and mitochondrial genetic analysis.Those with eight tentacles are called octocorallia or Alcyonaria and comprise soft corals, sea fans and sea pens. Those with more than eight in a multiple of six are called hexacorallia or Zoantharia. This group includes reef-building corals (Scleractinians), sea anemones and zoanthids.

Anatomy

Initially believed to be a plant, William Herschel used a microscope to establish in the 18th Century that Coral had the characteristic thin cell membranes of an animal.
While a coral head appears to be a single organism, it is actually a group of many individual, yet genetically identical, polyps. The polyps are multicellular organisms. Polyps are usually a few millimeters in diameter, and are formed by a layer of outer epithelium and inner jellylike tissue known as the mesoglea. They are radially symmetrical with tentacles surrounding a central mouth, the only opening to the stomach or coelenteron, through which food is ingested and waste expelled.
The stomach closes at the base of the polyp, where the epithelium produces an exoskeleton called the basal plate or calicle (L. small cup). The calicle is formed by a thickened calcareous ring (annular thickening) with six supporting radial ridges (as shown below). These structures grow vertically and project into the base of the polyp. When a polyp is physically stressed, its tentacles contract into the calyx so that virtually no part is exposed above the skeletal platform. This protects the organism from predators and the elements.
The polyp grows by extension of vertical calices which occasionally septate to form a new, higher, basal plate. Over many generations this extension forms the large calcareous structures of corals and ultimately coral reefs.
Formation of the calcareous exoskeleton involves deposition of the mineral aragonite by the polyps from calcium and carbonate ions they acquire from seawater. The rate of deposition, while varying greatly across species and environmental conditions, can be as much as 10 g / m² of polyp / day (0.3 ounce / sq yd / day). This is light dependent, with night-time production 90% lower than that during the middle of the day.
Nematocysts are stinging cells at the tips of the calices that carry poison which they rapidly release in response to contact with another organism. The tentacles also bear a contractile band of epithelium called the pharynx. Jellyfish and sea anemones also carry nematocysts.
The polyps interconnect by a complex and well developed system of gastrovascular canals allowing significant sharing of nutrients and symbiotes. In soft corals these range in size from 50–500 micrometres (0.0020–0.020 in) in diameter and allow transport of both metabolites and cellular components.
Many corals as well as other cnidarian groups such as sea anemones (e.g. Aiptasia), form a symbiotic relationship with a class of algae, zooxanthellae, of the genus Symbiodinium. Aiptasia, while considered a pest among coral reef aquarium hobbyists, serves as a valuable model organism in the study of cnidarian-algal symbiosis. Typically a polyp harbors one species of algae. Via photosynthesis, these provide energy for the coral, and aid in calcification.The algae benefit from a safe environment, and consume the carbon dioxide and nitrogenous waste produced by the polyp. Due to the strain the algae can put on the polyp, stress on the coral often drives the coral to eject the algae. Mass ejections are known as coral bleaching, because the algae contribute to coral's brown coloration; other colors, however, are due to host coral pigments, such as GFPs (green fluorescent protein). Ejection increases the polyp's chances of surviving short-term stress—they can regain algae at a later time. If the stressful conditions persist, the polyp eventually dies.

Feeding

Polyps feed on a variety of small organisms, from microscopic plankton to small fish. The polyp's tentacles immobilize or kill prey using their nematocysts. The tentacles then contract to bring the prey into the stomach. Once digested, the stomach reopens, allowing the elimination of waste products and the beginning of the next hunting cycle.
These poisons are usually too weak to harm humans. An exception is fire coral.

Reproduction

Corals can be both gonochoristic (unisexual) and hermaphroditic, each of which can reproduce sexually and asexually. Reproduction also allows coral to settle new areas.

Sexual

Corals predominantly reproduce sexually. 25% of hermatypic corals (stony corals) form single sex (gonochoristic) colonies, while the rest are hermaphroditic.About 75% of all hermatypic corals "broadcast spawn" by releasing gametes—eggs and sperm—into the water to spread offspring. The gametes fuse during fertilization to form a microscopic larva called a planula, typically pink and elliptical in shape. A typical coral colony form several thousand larvae per year to overcome the odds against formation of a new colony.
Planulae exhibits positive phototaxis, swimming towards light to reach surface waters where they drift and grow before descending to seek a hard surface to which it can attach and establish a new colony. They also exhibit positive sonotaxis, moving towards sounds that emanate from the reef and away from open water. High failure rates afflict many stages of this process, and even though millions of gametes are released by each colony very few new colonies form. The time from spawning to settling is usually 2-3 days, but can be up to 2 months.The larva grows into a polyp and eventually becomes a coral head by asexual budding and growth.
Synchronous spawning is very typical on the coral reef and often, even when multiple species are present, all corals spawn on the same night. This synchrony is essential so that male and female gametes can meet. Corals must rely on environmental cues, varying from species to species, to determine the proper time to release gametes into the water. The cues involve lunar changes, sunset time, and possibly chemical signalling.Synchronous spawning may form hybrids and is perhaps involved in coral speciation.In some places the spawn can be visually dramatic, clouding the usually clear water with gametes, typically at night.
Corals use two methods for sexual reproduction, which differ in whether the female gametes are released:
·         Broadcasters, the majority of which mass spawn, rely heavily on environmental cues, because they release both sperm and eggs into the water. The corals use long-term cues such as day length, water temperature, and/or rate of temperature change. The short-term cue is most often the lunar cycle, with sunset cuing the release.About 75% of coral species are broadcasters, the majority of which are hermatypic, or reef-building corals.The positively buoyant gametes float towards the surface where fertilization produces planula larvae. The larvae swim towards the surface light to enter into currents, where they remain usually for two days, but can be up to three weeks, and in one known case two months,after which they settle and metamorphose into polyps and form colonies.
·         Brooders are most often ahermatypic (non-reef building) in areas of high current or wave action. Brooders release only sperm, which is negatively buoyant, and can harbor unfertilized eggs for weeks, lowering the need for mass synchronous spawning events, which do sometimes occur.After fertilization the corals release planula larvae which are ready to settle.

Asexual

Within a coral head the genetically identical polyps reproduce asexually, either via gemmation (budding) or division, both shown in the photo of Orbicella annularis. Budding involves a new polyp growing from an adult, whereas division forms two polyps each as large as the original.
·         Budding expands colony size. It occurs when a new corallite grows out from an adult polyp. As the new polyp grows it produces its body parts. The distance between the new and adult polyps grows, and with it the coenosarc (the common body of the colony; see coral anatomy). Budding can be:
o    Intra-tentacular—from its oral discs, producing same-sized polyps within the ring of tentacles.
o    Extra-tentacular—from its base, producing a smaller polyp.
·         Longitudinal division begins when a polyp broadens and then divides its coelenteron. The mouth also divides and new tentacles form. The two "new" polyps then generate their missing body parts and exoskeleton.
·         Transversal division occurs when polyps and the exoskeleton divide transversally into two parts. This means that one has the basal disc (bottom) and the other has the oral disc (top). The two new polyps must again generate the missing pieces.

Colony division

·         Fission occurs in some corals, especially among the family Fungiidae, where the colony splits into two or more colonies during early developmental stages.
Whole colonies can reproduce asexually through fragmentation or bailout, forming another individual colony with the same genotype.
·         Bailout occurs when a single polyp abandons the colony and settles on a different substrate to create a new colony.
·         Fragmentation, involves individuals broken from the colony during storms or other situations. The separated individuals can start new colonies.

Reefs

The hermatypic, stony corals are often found in coral reefs, large calcium carbonate structures generally found in shallow, tropical water. Reefs are built up from coral skeletons and held together by layers of calcium carbonate produced by coralline algae. Reefs are extremely diverse marine ecosystems hosting over 4,000 species of fish, massive numbers of cnidarians, mollusks, crustaceans, and many other animals.

Types

Perforate corals

Corals can be perforate or imperforate. Perforate corals have porous skeletons, which allows their polyps to connect with each other through the skeleton. Imperforate corals have hard solid skeletons.

Hermatypic corals

Hermatypic or stony corals build reefs. With the help of zooxanthellae, they convert surplus food to calcium carbonate forming a hard skeleton. Hermatypic species include Scleractinia, Millepora, Tubipora and Heliopora.
In the Caribbean alone 50 species of uniquely structured hard coral exist. Well known types include:
·         Brain coral grow to 1.8 meters (6 ft) in width.
·         Acropora and Staghorn coral grow fast and large and are important reef-builders. Staghorn coral displays large antler-like branches and grows in areas with strong surf.
·         Galaxea fascicularis or star coral is another important reef-builder.
·         Pillar coral forms pillars which can grow to 3 meters (10 ft) in height.
·         Leptopsommia or rock coral, appears almost everywhere in the Caribbean.

Ahermatypic corals

Ahermatypic corals have no zooxanthellae and do not build reefs. They include Alcyonaceas, as well as some Anthipatharia-species (Black coral, Cirripathes, Antipathes). Ahermatypic corals such as sea whips, sea feathers, and sea pens are also known as soft corals. Unlike stony corals, they are flexible, undulating back and forth in the current, and often are perforated, with a lacy appearance. Their skeletons are proteinaceous, rather than calcareous. Soft corals are somewhat less plentiful (in the Caribbean, twenty species appear) than stony corals.

Evolutionary history

Although corals first appeared in the Cambrian period, some 542 million years ago, fossils are extremely rare until the Ordovician period, 100 million years later, when Rugose and Tabulate corals became widespread.
Tabulate corals occur in the limestones and calcareous shales of the Ordovician and Silurian periods, and often form low cushions or branching masses alongside Rugose corals. Their numbers began to decline during the middle of the Silurian period and they finally became extinct at the end of the Permian period, 250 million years ago. The skeletons of Tabulate corals are composed of a form of calcium carbonate known as calcite.
Rugose corals became dominant by the middle of the Silurian period, and became extinct early in the Triassic period. The Rugose corals existed in solitary and colonial forms, and are also composed of calcite.
The Scleractinian corals filled the niche vacated by the extinct Rugose and Tabulate species. Their fossils may be found in small numbers in rocks from the Triassic period, and become common in the Jurassic and later periods. Scleractinian skeletons are composed of a form of calcium carbonate known as aragonite. Although they are geologically younger than the Tabulate and Rugose corals, their aragonitic skeleton is less readily preserved, and their fossil record is less complete.
At certain times in the geological past corals were very abundant. Like modern corals, these ancestors built reefs, some of which now lie as great structures in sedimentary rocks.
Fossils of fellow reef-dwellers algae, sponges, and the remains of many echinoids, brachiopods, bivalves, gastropods, and trilobites appear along with coral fossils. This makes some corals useful index fossils, enabling geologists to date the age the rocks in which they are found.
Coral fossils are not restricted to reef remnants, and many solitary corals may be found elsewhere, such as Cyclocyathus, which occurs in England's Gault clay formation.
A Petoskey stone is a rock and a fossil, often pebble-shaped, that is composed of a fossilized coral, Hexagonaria percarinata. They are found predominantly in Michigan's Upper Peninsula, and the northwestern portion of Michigan's lower peninsula.

Threats

Corals are highly sensitive to environmental changes. Scientists have predicted that over 50% of the world's coral reefs may be destroyed by 2030; as a result most nations protect them through environmental laws.
Seaweed/Algae can destroy a coral reef. In the Caribbean and tropical Pacific, direct contact between ~40 to 70% of common seaweeds and coral cause bleaching and death to the coral via transfer of lipid–soluble metabolites.Seaweed and algae proliferate given adequate nutrients and limited grazing by herbivores. Coral die if surrounding water temperature changes by more than a degree or two beyond their normal range or if water salinity drops. In an early symptom of environmental stress, corals expel their zooxanthellae; without their symbiotic algae, coral tissues become colorless as they reveal the white of their calcium carbonate skeletons, an event known as coral bleaching.
Many governments now prohibit removal of coral from reefs and use education to inform their populations about reef protection and ecology. However, many other human activities damage reefs, including runoff, mooring, fishing, diving, mining and construction.
Coral's narrow niche and the stony corals' reliance on calcium carbonate deposition makes them susceptible to changes in water PH. The increase in atmospheric carbon dioxide has caused enough dissolution of carbon dioxide to lower the ocean's pH, in a process known as ocean acidification. Lowered pH reduces corals' ability to produce calcium carbonate, and at the extreme, can dissolve their skeletons. Without deep and immediate cuts in anthropogenic CO2, many scientists fear that acidification will severely degrade or destroy coral ecosystems.

Importance to humans

Local economies near major coral reefs benefit from an abundance of fish and other marine creatures as a food source. Reefs also provide recreational scuba diving and snorkeling tourism. Unfortunately these activities can have deleterious effects, such as accidental destruction of coral. Coral is also useful as a protection against hurricanes and other extreme weather.
Coral reefs provide medical benefits for humans. Chemical compounds taken from corals are used in medicine for cancer, AIDS, pain, and other uses. Corals are also commonly used for bone grafting in humans.
Live coral is highly sought after for aquaria. Given the proper ecosystem, live coral makes a stunning addition to any salt water aquarium. Soft corals are easier to maintain in captivity than hard corals.
 Isididae may be usable as living bone implants and in aquatic cultivation, because of their potential to mimic valuable biological properties.

In jewelry

Coral's many colors give it appeal for necklaces and other jewelry. Intensely red coral is prized as a gemstone. It is sometimes called fire coral, but is not the same as fire coral. Red coral is very rare because of overharvesting due to the great demand for perfect specimens.

In construction

Ancient coral reefs on land provide lime or use as building blocks ("coral rag"). Coral rag is an important local building material in places such as the East African coast.

In climate research

The annual growth bands in bamboo corals and others allow geologists to construct year-by-year chronologies, a form of incremental dating, which underlie high-resolution records of past climatic and environmental changes using geochemical techniques.
Certain species form communities called microatolls, which are colonies whose top is dead and mostly above the water line, but whose perimeter is mostly submerged and alive. Average tide level limits their height. By analyzing the various growth morphologies, microatolls offer a low resolution record of sea level change. Fossilized microatolls can also be dated using radioactive carbon dating. Such methods can help to reconstruct Holocene sea levels.
Deep sea bamboo corals (Isididae) may be among the first organisms to display the effects of ocean acidification. They produce growth rings similar to those of tree and can provide a view of changes in the condition in the deep sea over time.
Tortoise Shell

Tortoiseshell material

Tortoiseshell or tortoise shell is a material produced mainly from the shell of the hawksbill turtle, an endangered species. It was widely used in the 1960s and 1970s in the manufacture of items such as combs, sunglasses, guitar picks and knitting needles. In 1973, the trade of tortoiseshell worldwide was banned under CITES (the Convention on International Trade in Endangered Species).
Tortoiseshell was attractive to manufacturers and consumers because of its beautiful appearance and its durability, and its organic warmth against the skin. It was used in guitar picks because it can be easily shaped, has excellent bending properties, and is very durable – tortoiseshell picks could sometimes be used for years. Pique work, jewelry made from tortoiseshell inlaid with precious metals in patterns or pictures, was made during the Victorian Era and was highly prized.
There have been a number of faux tortoiseshell materials developed since the 1970s, most of which mimic the appearance of tortoiseshell. Tortex is a material that was created to replace tortoiseshell guitar picks and has been widely accepted as a suitable substitute.
Shell
http://upload.wikimedia.org/wikipedia/commons/d/d4/Button_hide.png

A seashell, also known as a sea shell, or simply as a shell, is the common name for a hard, protective outer layer, a shell, or in some cases a "test", that was created by a sea creature, a marine organism. The shell is part of the body of a marine animal. In most cases a shell is an exoskeleton, usually that of an animal without a backbone, an invertebrate. Seashells are most often found washed up empty on beaches or another part of the coastline after the soft parts of the animal have either been eaten by another animal that attacked it (predation), or after the animal has died and the soft parts have been eaten by scavengers or have simply rotted out.
The word seashell is most often used to mean the shells of marine mollusks, i.e. mollusk shells. It can however also be used to mean the shells of a wide variety of other marine animals from various different marine invertebrates.
As well as marine mollusks, many other kinds of sea animals have exoskeletons or even internal shells which sometimes, after death, wash up on the beach and may be picked up by beachcombers. These shells include remains from species in other invertebrate phyla, such as the moulted shells or exuviae of crabs and lobsters, the shells of barnacles, horseshoe crab shells, the tests (endoskeletons) of sea urchins, sand dollars and seastars, brachiopod shells, and the shells of marine annelid worms in the family Serpulidae, which create calcareous tubes cemented onto other surfaces.
Seashells have been admired, studied and used by humans for many different purposes throughout history and pre-history.
Sea shells are not the only kind of shells, in a variety of habitats it is possible to find shells from freshwater animals such as freshwater mussels and freshwater snails, and it is also possible to find shells from land snails.
Terminology
When the word "seashells" is used to refer only to the shells of marine mollusks, (spelled "molluscs" in the UK), then studying seashells is part of conchology. Conchologists or serious collectors who have a scientific bias are in general careful not to disturb living populations and habitats: even though they may collect a few live animals, most responsible collectors do not often over-collect or otherwise disturb ecosystems.
When studying the whole molluscan animal is included as well as studying the shell, then the study is known as malacology; a person who studies mollusks is known as a malacologist.
Finding seashells
Seashells are commonly found in beach drift, which is natural detritus deposited along strandlines on beaches by the waves and the tides. Shells are very often washed up onto a beach empty and clean, the animal having already died, and the soft parts having rotted away or having been eaten by either predators or scavengers.
Empty seashells are often picked up by beachcombers, and collecting these shells is a harmless hobby or study. However, the majority of seashells which are offered for sale commercially have been collected alive (often in bulk) and then killed and cleaned, specifically for the commercial trade. This type of large-scale exploitation can sometimes have a strong negative impact on local ecosystems, and sometimes can significantly reduce the distribution of rare species.
 Molluscan seashells
The word "seashells" is often used to mean only the shells of marine mollusks. Marine mollusk shells that are familiar to beachcombers and thus most likely to be called "seashells" are the shells of marine species of bivalves (or clams), gastropods (or snails), scaphopods (or tusk shells), polyplacophorans (or chitons), and cephalopods (such as nautilus and spirula). These shells are very often the most commonly encountered, both in the wild, and for sale as decorative objects.
Marine species of gastropods and bivalves are more numerous than land and freshwater species, and the shells are often larger and more robust. The shells of marine species also often have more sculpture and more color, although this is by no means always the case.
In the tropical and sub-tropical areas of the planet, there are far more species of colorful, large, shallow water shelled marine mollusks than there are in the temperate zones and the regions closer to the poles.
Although there are a number of species of shelled mollusks that are quite large, there are vast numbers of extremely small species too, see micromollusks.
Not all mollusks are marine however, there are numerous land and freshwater mollusks, see for example snail and freshwater bivalves. And not all mollusks have an external shell: some mollusks such as some cephalopods (squid and octopuses) have an internal shell, and many mollusks have no shell, see for example slug and nudibranch.
Bivalves
Bivalves are often the most common seashells that wash up on large sandy beaches or in sheltered lagoons. They can sometimes be extremely numerous. Very often the two valves become separated.
Shell Beach, Western Australia is a beach which is entirely made up of the shells of the cockle Fragum erugatum as shown here.
 Gastropods
Certain species of gastropod seashells (the shells of sea snails) can sometimes be common, washed up on sandy beaches, and also on beaches that are surrounded by rocky marine habitat.
 Polyplacophorans
Chiton plates or valves often wash up on beaches in rocky areas where chitons are common. Chiton shells, which are composed of eight separate plates and a girdle, usually come apart not long after death, so they are almost always found as disarticulated plates. Plates from larger species of chitons are sometimes known as "butterfly shells" because of their shape.
 Cephalopods
Only a few species of cephalopods have shells (either internal or external) that are sometimes found washed up on beaches.
Some cephalopods such as Sepia, the cuttlefish, have a large internal shell, the cuttlefish bone, and this often washes up on beaches in parts of the world where cuttlefish are common.
Spirula spirula is a deep water squid-like cephalopod. It has an internal shell which is small (about 1 in or 24 mm) but very light and buoyant. This chambered shell floats very well and therefore washes up easily and is familiar to beachcombers in the tropics.
Nautilus is the only genus of cephalopod that has a well-developed external shell. Females of the cephalopod genus Argonauta create a papery egg case which sometimes washes up on tropical beaches and is referred to as a "paper nautilus".
The largest group of shelled cephalopods, the ammonites, are extinct, but their shells are very common in certain areas as fossils.
Molluscan seashells used by other animals
Empty molluscan seashells are a sturdy, and usually readily available, "free" resource which is often easily found on beaches, in the intertidal zone, and in the shallow subtidal zone. As such they are sometimes used second-hand by animals other than humans for various purposes, including for protection (as in hermit crabs) and for construction.
 By other mollusks
·         Carrier shells in the family Xenophoridae are marine shelled gastropods, fairly large sea snails. Most species of xenophorids cement a series of objects to the rim of their shells as they grow. These objects are sometimes small pebbles or other hard detritus. Very often shells of bivalves or smaller gastropods are used, depending on what is available on the particular substrate where the snail itself lives. It is not clear whether these shell attachments serve as camouflage, or whether they are intended to help prevent the shell sinking into a soft substrate.
·         Small octopuses sometimes use an empty shell as a sort of cave to hide in, or hold seashells around themselves as a form of protection like a temporary fortress.
By other invertebrates
·         Almost all genera of hermit crabs use or "wear" empty marine gastropod shells throughout their lifespan, in order to protect their soft abdomens, and in order to have a strong shell to withdraw into if attacked by a predator. Each individual hermit crab is forced to find another gastropod shell on a regular basis, whenever it grows too large for the one it is currently using.
Some hermit crab species live on land and may be found quite some distance from the sea, including those in the tropical genus Coenobita.
 Human uses
 Collecting shells as a hobby and a study
There are numerous popular books and field guides on the subject of shell-collecting. Although there are a number of books about land and freshwater mollusks, the majority of popular books emphasize, or focus exclusively on, the shells of marine mollusks.
Both the science of studying mollusk shells and the hobby of collecting and classifying them are known as conchology. The line between professionals and amateur enthusiasts is often not well defined in this subject, because many amateurs have contributed to, and continue to contribute to, conchology and the larger science of malacology. Many shell collectors belong to "shell clubs" where they can meet others who share their interests.
A large number of amateurs collect the shells of marine mollusks, and this is partly because many shells wash up empty on beaches, or live in the intertidal or sub-tidal zones, and are therefore easily found and preserved without much in the way of specialized equipment or expensive supplies.
Some shell collectors find their own material and keep careful records, or buy only "specimen shells", which means shells which have full collecting data: information including how, when, where, in what habitat, and by whom, the shells were collected. On the other hand, some collectors buy the more widely available commercially-imported exotic shells, the majority of which have very little data, or none at all.
To museum scientists, having full collecting data (when, where, and by whom it was collected) with a specimen is far more important than having the shell correctly identified. Some owners of shell collections hope to be able to donate their collection to a major natural history or zoology museum at some point, however, shells with little or no collecting data are usually of no value to science, and are likely not to be accepted by a major museum.
Apart from any damage to the shell that may have happened before it was collected, shells can also suffer damage when they are stored or displayed. For an example of one rather serious kind of damage see Byne's disease.
 Shell clubs
There are a number of clubs or societies which consist of people who are united by a shared interest in shells. In the USA these clubs are more common in southerly coastal areas, such as Florida and California, where the marine fauna is rich in species.
 Identification
Seashells are usually identified by consulting general or regional shell-collecting field guides, and specific scientific books on different taxa of shell-bearing mollusks (monographs) or "iconographies" (limited text - mainly photographs or other illustrations). (For a few titles on this subject in the USA, see the list of books at the foot of this article.)
Identifications to the species level are generally achieved by examining illustrations and written descriptions, rather than by the use of Identification keys, as is often the case in identifying plants and other phyla of invertebrates. The construction of functional keys for the identification of the shells of marine mollusks to the species level can be very difficult, because of the great variability within many species and families.
The identification of certain individual species is often very difficult, even for a specialist in that particular family. Some species cannot be differentiated on the basis of shell character alone.
Numerous smaller and more obscure mollusk species (see micromollusk) are yet to be discovered and named. In other words, they have not yet been differentiated from similar species and assigned scientific (binomial) names in articles in journals recognized by the International Commission on Zoological Nomenclature (ICZN). Large numbers of new species are published in the scientific literature each year. There are currently an estimated 100,000 species of mollusks worldwide.
 Non-marine and marine shell confusion
Because virtually all rivers discharge into the sea, and because heavy rain can carry land snail shells into rivers, the shells of freshwater snails and freshwater clams, and shells of land snails, can all sometimes wash up onto saltwater beaches, mixed with the shells of marine species. This can be confusing to shell collectors who are attempting to identify what they have found.
When seashells are purchased from shops or dealers, it is common to encounter various non-marine shells for sale as well. Sometimes the non-marine shells are mixed in with the marine shells. The non-marine items often include solid and colorful shells such certain tropical land snail shells, freshwater apple snail shells, and pearly freshwater unionid mussel shells. This also can be confusing to collectors.
 Significance of molluscan seashells in human culture

As Additive to poultry feeds

Sea shells are found in the creek and backwater of the coast of west india is used as a additive to the poultry feed. It is crushed and mixed with jawar maaze and dry fish.
 As currency
Seashells have been used as a medium of exchange in various places, including many Indian Ocean and Pacific Ocean islands, also in North America, Africa and the Caribbean.
·         The most common species of shells to be used as currency have been Cypraea moneta, the “money cowry”, and certain tusk shells or dentalium, such as those used in North Western North America for many centuries.
·         Some tribes of the indigenous peoples of the Americas used shells for wampum and hair pipes.The Native American wampum belts were made of the shell of the quahog clam.
·         It is of historic interest that the Dutch East India Company, a major force in the colonization of the Indian and Pacific Oceans, amassed a large portion of its vast fortune via trading shell money of the species Cypraea moneta and Cypraea annulus, in exchange for commodities such as spices, exotic animals, and gemstones, all of which were considered valuable in Europe at the time.
 As tools
Seashells have often been used as tools, because of their strength and the variety of their shapes.
·         Giant clams (Family Tridacnidae) have been used as bowls, and when big enough, even as bathtubs and baptismal fonts.
·         Melo melo, the "bailer volute", is so named because Native Australians used it to bail out their canoes.
·         Many different species of bivalves have been used as scrapers, blades, clasps, and other such tools, due to their shape.
·         Some marine gastropods have been used for oil lamps, the oil being poured in the aperture of the shell, and the siphonal canal serving as a holder for the wick.
 In horticulture
Because shells are sometimes a readily available bulk source of calcium carbonate, shells such as oyster shells are sometimes used as soil conditioners in horticulture. The shells are broken or ground into small pieces in order to have the desired effect of raising the pH and increasing the calcium content in the soil.
 In religion and spirituality
Seashells have played a part in religion and spirituality, sometimes even as ritual objects.
·         In Christianity, the scallop shell is considered to be the symbol of Saint James the Great, see Pecten jacobaeus.
·         In Hinduism left-handed shells of Turbinella pyrum (the sacred shankha) are considered to be sacred to the god Vishnu. The person who finds a left-handed chank shell (one that coils to the left) is sacred to Vishnu, as well. The chank shell also plays an important role in Buddhism.
·         Cowries have often been considered to be symbols of female fertility. They were often treated as actual fertility charms. The dorsum of the shell resembles a pregnant belly, and the underside of the shell resembles a vulva. In the South Indian state of Kerala, cowries are used for making astrological predictions.
·         In Santeria, shells are used for divination purposes.
·         The Moche culture of ancient Peru worshipped animals and the sea, and often depicted shells in their art.
 As musical instruments
Seashells have been used as musical instruments, wind instruments for many hundreds if not thousands of years. Most often the shells of large sea snails are used, as trumpets, by cutting a hole in the spire of the shell, or cutting off the tip of the spire altogether.
Various different kinds of large marine gastropod shells can be turned into "blowing shells", however the most commonly encountered species used as "conch" trumpets are:
·         The sacred chank, Turbinella pyrum, known in India as the shankha. In Tibet it is known as "dung-dkar".
·         The Triton shell also known as "Triton's trumpet" Charonia tritonis which is used as a trumpet in Melanesian and Polynesian culture and also in Korea and Japan. In Japan this kind of trumpet is known as the horagai. In Korea it is known as the nagak. In some Polynesian islands it is known as "pu".
·         The Queen Conch Strombus gigas, is or was sometimes used as a trumpet in the Caribbean.
 In personal adornment
Whole seashells or parts of sea shells have been used as jewelry or in other forms of adornment since prehistoric times. Mother of pearl was historically primarily a seashell product, although more recently some mother of pearl comes from freshwater mussels. Also see pearl.
·         Shell necklaces have been found in Stone Age graves as far inland as the Dordogne Valley in France.
·         Seashells are often used whole and drilled, so that they can be threaded like beads, or cut into pieces of various shapes. Sometimes shells can be found that are already "drilled" by predatory snails of the family Naticidae.
·         Naturally-occurring, beachworn, cone shell "tops" (the broken-off spire of the shell, which often has a hole worn at the tip) can function as beads without any further modification. In Hawaii these natural beads were traditionally collected from the beach drift in order to make puka shell jewelry. Since it is hard to obtain large quantities of naturally-occurring beachworn cone tops, almost all modern puka shell jewelry uses cheaper imitations, cut from thin shells of other species of mollusk, or even made of plastic.
·         Shells historically have been and still are made into, or incorporated into, necklances, pendants, beads, earrings, buttons, brooches, rings, hair combs, belt buckles and other uses.
·         The shell of the large "bullmouth helmet" sea snail, scientific name Cypraecassis rufa, was historically, and still is, used to make valuable cameos.
·         Mother of pearl from many seashells including species in the family Trochidae, Turbinidae, Haliotidae, and various pearly bivalves, has often been used in jewelry, buttons, etc.
·         In London, Pearly Kings and Queens traditionally wear clothing covered in patterns made up of hundreds of "pearl buttons", in other words, buttons made of mother-of-pearl or nacre. In recent years however, the majority of "pearl buttons" are imitations that are made of pearlescent plastic.
 In crafts
"Sailor's Valentines" were late 19th century decorative keepsakes which were made in the Caribbean, and which were often purchased by sailors to give to their loved ones back home for example in England. These valentines consisted of elaborate arrangements of small seashells glued into attractive symmetrical designs, which were encased on a wooden (usually octagonal) hinged box-frame. The patterns used often featured heart-shaped designs, or included a sentimental expression of love spelled out in small shells.
 In architectural decoration
Small pieces of colored and iridescent shell have been used to create mosaics and inlays, which have been used to decorate walls, furniture and boxes.
Large numbers of whole seashells, arranged to form patterns, have been used to decorate mirror frames, furniture and man-made grottos.
 In art
The pleasing designs of seashells have caused them to be featured in art in various ways, in paintings, in sculpture, and so on.
A very large outdoor sculpture at Akkulam of a gastropod seashell is a reference to the sacred chank shell Turbinella pyrum of India.
Maggi Hambling designed a striking 13 ft (4 m) high sculpture of a scallop shell which stands on the beach at Aldeburgh, in England.
The goddess of love, Venus or Aphrodite is often traditionally depicted rising from the sea on a seashell.
In the Birth of Venus (Botticelli), Botticelli depicted the goddess Venus rising from the ocean on a scallop shell.
Shells of other marine invertebrates
 Arthropods
Many arthropods have sclerites, or hardened body parts, which form a stiff exoskeleton made up mostly of chitin. In crustaceans, especially those of the class Malacostraca (crabs, shrimps and lobsters, for instance), the plates of the exoskeleton may be fused to form a more or less rigid carapace. Moulted carapaces of a variety of marine malacostraceans often wash up on beaches.
The horseshoe crab is an arthropod of the family Limulidae. The shells or exuviae of these arachnids are common in beach drift in certain areas of the world.
 Sea urchins
Some echinoderms such as sea urchins, including heart urchins and sand dollars, have a hard "test" or shell. After the animal dies, the flesh rots out and the spines fall off, and then fairly often the empty test washes up whole onto a beach, where it can be found by a beachcomber. These tests are fragile and easily broken into pieces.
 Brachiopods
The brachiopods, or lamp shells, superficially resemble clams, but the phylum is completely unrelated to mollusks. Most lines of brachiopods ended during the Permian-Triassic extinction event, and their ecological niche was filled by bivalves. A few of the remaining species of brachiopods occur in the low intertidal zone and thus can be found live by beachcombers.
 Annelids
Some polychaetes, marine annelid worms in the family Serpulidae, secrete a hard tube made of calcium carbonate, adhering to stones or other shells. This tube resembles, and can be confused with, the shell of marine gastropod mollusks in the family Vermetidae, the worm snails.
 Other more atypical kinds
A few other categories of marine animals leave remains which might be considered "seashells" in the widest possible sense of the word.
 Vertebrate "shells": chelonians
Sea turtles have a carapace and plastron of bone and cartilage which is developed from their ribs. Infrequently a turtle "shell" will wash up on a beach.
 Hard corals
Pieces of the hard skeleton of corals commonly wash up on beaches in areas where corals grow.
The construction of the shell-like structures of corals are aided by a symbiotic relationship with a class of algae, zooxanthellae. Typically a coral polyp will harbour particular species of algae, which will photosynthesise and thereby provide energy for the coral and aid in calcification,while living in a safe environment and using the carbon dioxide and nitrogenous waste produced by the polyp. Coral bleaching is a disruption of the balance between polyps and algae, and can lead to the breakdown and death of coral reefs.
 Soft corals
The skeletons of soft corals such as gorgonians, also known as sea fans and sea whips, commonly wash ashore in the tropics after storms.
 Plankton and protists
Plant-like diatoms and animal-like radiolarians are two forms of plankton which form hard silicate shells. Foraminifera and coccolithophore create shells known as "tests" which are made of calcium carbonate. All these shells and tests are usually (but in the case of foraminifera not always) microscopic in size.
Pearl
A pearl is a hard object produced within the soft tissue (specifically the mantle) of a living shelled mollusk. Just like the shell of a mollusk, a pearl is made up of calcium carbonate in minute crystalline form, which has been deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other shapes of pearls (baroque pearls) occur. The finest quality natural pearls have been highly valued as gemstones and objects of beauty for many centuries, and because of this, the word pearl has become a metaphor for something very rare, fine, admirable, and valuable.
The most valuable pearls occur spontaneously in the wild, but they are extremely rare. Cultured or farmed pearls from pearl oysters make up the majority of those that are currently sold. Pearls from the sea are valued more highly than freshwater pearls. Imitation or fake pearls are also widely sold in inexpensive jewelry, but the quality of their iridescence is usually very poor, and generally speaking, artificial pearls are easily distinguished from genuine pearls. Pearls have been harvested and cultivated primarily for use in jewelry, but in the past they were also stitched onto lavish clothing. Pearls have also been crushed and used in cosmetics, medicines, and in paint formulations.
Pearls that are considered to be of gemstone quality are almost always nacreous and iridescent, wild or cultured, like the interior of the shell that produces them. However, almost all species of shelled mollusks are capable of producing pearls (formerly referred to as "calcareous concretions" by some sources) of lesser shine or less spherical shape. Although these may also be legitimately referred to as "pearls" by gemological labs and also under U.S. Federal Trade Commission rules,and are formed in the same way, most of them have no value, except as curios.

Etymology

The English word pearl originated (via French perle) from the Latin word perla. It is used to translate the Hebrew word גוו בייש of the Old Testament, mar-gar-ee-tare, which means pearl in Greek (the English name Margaret originated from the Greek word for pearl).

Definition

Almost any shelled mollusk can, by natural processes, produce some kind of "pearl" when an irritating microscopic object becomes trapped within the mollusk's mantle folds, but the great majority of these "pearls" are not valued as gemstones. Nacreous pearls, the best-known and most commercially-significant pearls, are primarily produced by two groups of molluscan bivalves or clams. A nacreous pearl is made from layers of nacre, by the same living process as is used in the secretion of the mother of pearl which lines the shell.
A "natural pearl" or "wild pearl" is one that forms without any human intervention at all, in the wild, and is very rare. Many hundreds of pearl oysters or pearl mussels have to be gathered and opened, and thus killed, in order to find even one wild pearl, and for many centuries that was the only way pearls were obtained. This was the main reason why pearls fetched such extraordinary prices in the past. A cultured pearl is formed in a pearl farm, using human intervention as well as natural processes.
One family of nacreous pearl bivalves – the pearl oyster – lives in the sea, while the other – a very different group of bivalves – lives in freshwater; these are the river mussels such as the freshwater pearl mussel. Saltwater pearls can grow in several species of marine pearl oysters in the family Pteriidae. Freshwater pearls grow within certain (but by no means all) species of freshwater mussels in the order Unionida, the families Unionidae and Margaritiferidae.

Physical properties

The unique luster of pearls depends upon the reflection, refraction, and diffraction of light from the translucent layers. The thinner and more numerous the layers in the pearl, the finer the luster. The iridescence that pearls display is caused by the overlapping of successive layers, which breaks up light falling on the surface. In addition, pearls (especially cultured freshwater pearls) can be dyed yellow, green, blue, brown, pink, purple, or black.

Freshwater and saltwater pearls

Freshwater and saltwater pearls may sometimes look quite similar, but they come from different sources.
Natural freshwater pearls form in various species of freshwater mussels, family Unionidae, which live in lakes, rivers, ponds and other bodies of fresh water. These freshwater pearl mussels occur not only in hotter climates, but also in colder more temperate areas such as Scotland: see the freshwater pearl mussel. However, most freshwater cultured pearls sold today come from China.
Saltwater pearls grow within pearl oysters, family Pteriidae, which live in oceans. Saltwater pearl oysters are usually cultivated in protected lagoons or volcanic atolls.

Creation of a pearl

The difference between wild and cultured pearls focuses on whether the pearl was created spontaneously by nature – without human intervention – or with human aid. Pearls are formed inside the shell of certain mollusks as a defense mechanism against a potentially threatening irritant such as a parasite inside its shell, or an attack from outside, injuring the mantle tissue. The mollusk creates a pearl sac to seal off the irritation.
The mantle of the mollusk deposits layers of calcium carbonate (CaCO3) in the form of the mineral aragonite or a mixture of aragonite and calcite (polymorphs with the same chemical formula, but different crystal structures) held together by an organic horn-like compound called conchiolin. The combination of aragonite and conchiolin is called nacre, which makes up mother-of-pearl. The commonly held belief that a grain of sand acts as the irritant is in fact rarely the case. Typical stimuli include organic material, parasites, or even damage that displaces mantle tissue to another part of the mollusk's body. These small particles or organisms gain entry when the shell valves are open for feeding or respiration. In cultured pearls, the irritant is typically an introduced piece of the mantle epithelium, together or without a spherical bead (beaded or beadless cultured pearls).

 Natural pearls

Natural pearls are nearly 100% calcium carbonate and conchiolin. It is thought that natural pearls form under a set of accidental conditions when a microscopic intruder or parasite enters a bivalve mollusk, and settles inside the shell. The mollusk, being irritated by the intruder, forms a pearl sac of external mantle tissue cells and secretes the calcium carbonate and conchiolin to cover the irritant. This secretion process is repeated many times, thus producing a pearl. Natural pearls come in many shapes, with perfectly round ones being comparatively rare.
Typically, the build-up of a natural pearl consists of a brown central zone formed by columnar calcium carbonate (usually calcite, sometimes columnar aragonite) and a yellowish to white outer zone consisting of nacre (tabular aragonite). In a pearl cross-section such as in Fig. 6, these two different materials can be seen. The presence of columnar calcium carbonate rich in organic material indicates juvenile mantle tissue that formed during the early stage of pearl development. Displaced living cells with a well-defined task may continue to perform their function in their new location, often resulting in a cyst. Such displacement may occur via an injury. The fragile rim of the shell is exposed and is prone to damage and injury. Crabs, other predators and parasites such as worm larvae may produce traumatic attacks and cause injuries in which some external mantle tissue cells are disconnected from their layer. Embedded in the conjunctive tissue of the mantle, these cells may survive and form a small pocket in which they continue to secrete their natural product: calcium carbonate. The pocket is called a pearl sack, and grows with time by cell division; in this way the pearl grows also. The juvenile mantle tissue cells, according to their stage of growth, produce columnar calcium carbonate, which is secreted from the inner surface of the pearl sack. With ongoing time the external mantle cells of the pearl sack proceed to the formation of tabular aragonite. When the transition to nacre secretion occurs, the brown pebble becomes covered with a nacreous coating. As this process progresses, the shell itself grows, and the pearl sack seems to travel into the shell. However, it actually stays in its original relative position within the mantle tissue. After a couple of years, a pearl will have formed and the shell might be found by a lucky pearl fisher.

Cultured pearls

Cultured pearls are the response of the shell on a tissue implant. A tiny piece of mantle tissue of a donor shell is transplanted into a recipient shell. This graft will form a pearl sac and the tissue will precipitate calcium carbonate into this pocket. There are a number of options for producing cultured pearls: use freshwater or seawater shells, transplant the graft into the mantle or into the gonad, add a spherical bead or do it non-beaded. The large majority of saltwater cultured pearls are grown with beads, the trade name of the cultured pearls are Akoya, white or golden South sea, black Tahiti. The majority of beadless cultured pearls are mantle-grown in freshwater shells, tradename Chinese cultured pearls.
Cultured pearls (beadless or beaded) and imitation pearls can be distinguished from natural pearls by X-ray examination. Nucleated cultured pearls are often 'pre-formed' as they tend to follow the shape of the implanted shell bead nucleus. Once the pre-formed beads are inserted into the oyster, it secretes a few layers of nacre around the outside surface of the implant before it is removed after six months or more.
When a cultured pearl with bead is X-rayed, it reveals a different structure to that of a natural pearl. A beaded cultured pearl shows a solid center with no concentric growth rings, whereas a natural pearl shows a series of concentric growth rings. A beadless cultured pearl (whether of freshwater or saltwater origin) may show growth rings, but also a complex central cavity, witness of the first precipitation of the young pearl sac.

 Gemological identification

A well equipped gem testing laboratory is able to distinguish natural pearls from cultured pearls by using a gemological x-ray in order to examine the center of a pearl. With an x-ray it is possible to see the growth rings of the pearl, where the layers of calcium carbonate are separated by thin layers of conchiolin. The differentiation of natural pearls from non-beaded cultured pearls can be very difficult without the use of this x-ray technique.
Natural and cultured pearls can be distinguished from imitation pearls using a microscope. Another method of testing for imitations is to rub two pearls against each other. Imitation pearls are completely smooth, but natural and cultured pearls are composed of nacre platelets, making both feel slightly gritty.

 Value of a natural pearl

Quality natural pearls are very rare jewels. The actual value of a natural pearl is determined in the same way as it would be for other "precious" gems. The valuation factors include size, shape, quality of surface, orient and luster.
Single natural pearls are often sold as a collector's item, or set as centerpieces in unique jewelry. Very few matched strands of natural pearls exist, and those that do often sell for hundreds of thousands of dollars. (In 1917, jeweler Pierre Cartier purchased the Fifth Avenue mansion that is now the New York Cartier store for US$100 cash and a double strand of matched natural pearls valued at the time at US$1 million.)
Keshi pearls, although they often occur by chance, are not considered natural pearls. They are a byproduct of the culturing process, and hence do not happen without human intervention. These pearls are quite small: typically a few millimeters in size. Keshi pearls are produced by many different types of marine mollusks and freshwater mussels in China.Today many "keshi" pearls are actually intentional, with post-harvest shells returned to the water to regenerate a pearl in the existing pearl sac.

 Origin of a natural pearl

Previously, natural pearls were found in many parts of the world. Present day natural pearling is confined mostly to seas off Bahrain. Australia also has one of the world's last remaining fleets of pearl diving ships. Australian pearl divers dive for south sea pearl oysters to be used in the cultured south sea pearl industry. The catch of pearl oysters is similar to the numbers of oysters taken during the natural pearl days. Hence significant numbers of natural pearls are still found in the Australian Indian Ocean waters from wild oysters. X-Ray examination is required to positively verify natural pearls found today.

Different types of cultured pearls, including black pearls

Black pearls, frequently referred to as Black Tahitian Pearls, are highly valued because of their rarity; the culturing process for them dictates a smaller volume output and can never be mass produced.This is due to bad health and/or non-survival of the process, rejection of the nucleus and their sensitivity to changing climatic and ocean conditions. Before the days of cultured pearls, black pearls were rare and highly valued for the simple reason that white pearl oysters rarely produced naturally black pearls, and black pearl oysters rarely produced any natural pearls at all.
Since the development of pearl culture technology, the black pearl oyster found in Tahiti and many other Pacific Island areas has been extensively used for producing cultured pearls. The rarity of the black cultured pearl is now a "comparative" issue. The black cultured pearl is rare when compared to Chinese freshwater cultured pearls, and Japanese and Chinese akoya cultured pearls, and is more valuable than these pearls. However, it is more abundant than the South Sea pearl, which is more valuable than the black cultured pearl. This is simply because the black pearl oyster Pinctada margaritifera is far more abundant than the elusive, rare, and larger south sea pearl oyster Pinctada maxima, which cannot be found in lagoons, but which must be dived for in a rare number of deep ocean habitats or grown in hatcheries.
Black cultured pearls from the black pearl oyster – Pinctada margaritifera – are not South Sea pearls, although they are often mistakenly described as black South Sea pearls. In the absence of an official definition for the pearl from the black oyster, these pearls are usually referred to as "black Tahitian pearls".
The correct definition of a South Sea pearl – as described by CIBJO and GIA – is a pearl produced by the Pinctada maxima pearl oyster. South Sea pearls are the color of their host Pinctada maxima oyster – and can be white, silver, pink, gold, cream, and any combination of these basic colors, including overtones of the various colors of the rainbow displayed in the pearl nacre of the oyster shell itself.

Pearls from other species

Biologically speaking, under the right set of circumstances, almost any shelled mollusk can produce some kind of pearl, however, most of these molluscan pearls have no luster or iridescence. The great majority of mollusk species produce pearls which are not attractive to look at, and are sometimes not even very durable, such that they usually have no value at all, except perhaps to a scientist, a collector, or as a curiosity. These objects used to be referred to as "calcareous concretions" by some gemologists, even though a malacologist would still consider them to be pearls. Valueless pearls of this type are sometimes found in edible mussels, edible oysters, escargot snails, and so on. The GIA and CIBJO now simply use the term 'pearl' (or, where appropriate, the more descriptive term 'non-nacreous pearl') when referring to such items and, under Federal Trade Commission rules, various mollusc pearls may be referred to as 'pearls' without qualification.
A few species produce pearls that can be of interest as gemstones. These species include the bailer shell Melo, the giant clam Tridacna, various scallop species, Pen shells Pinna, and abalones. Another example is the conch pearl (sometimes referred to simply as the 'pink pearl'), which is found very rarely growing between the mantle and the shell of the queen conch or pink conch, Strombus gigas, a large sea snail or marine gastropod from the Caribbean Sea. These pearls, which are often pink in color, are a by-product of the conch fishing industry, and the best of them display a shimmering optical effect related to chatoyance known as 'flame structure'.
Somewhat similar gastropod pearls, this time more orange in hue, are (again very rarely) found in the horse conch Pleuroploca gigantea.
The largest pearl known was found in the Philippines in 1934 and is known as the pearl of Lao Tzu. It is a naturally-occurring, non-nacreous, calcareous concretion (pearl) from a giant clam. Because it did not grow in a pearl oyster it is not pearly; instead the surface is glossy like porcelain. Other pearls from giant clams are known to exist, but this is a particularly large one, weighing 14 lb (6.4 kg).

The history of pearl hunting and pearl farming

Pearl hunting

For thousands of years, most seawater pearls were retrieved by divers working in the Indian Ocean, in areas like the Persian Gulf, the Red Sea, and in the Gulf of Mannar.Starting in the Han Dynasty (206 BC–220 AD), the Chinese hunted extensively for seawater pearls in the South China Sea. In the 14th-century Arabian Sea, the traveller Ibn Battuta provided the earliest known description of pearl diving by means of attaching a cord to the diver's waist.
When Spanish conquistadors arrived in the Western Hemisphere, they discovered that around the islands of Cubagua and Margarita, some 200 km north of the Venezuelan coast, was an extensive pearl bed (a bed of pearl oysters). One discovered and named pearl, La Peregrina pearl, was offered to the Spanish queen. According to Garcilasso de la Vega, who says that he saw La Peregrina at Seville in 1507, (Garcilasso, "Historie des Incas, Rois du Perou," Amsterdam, 1704, Vol. II, P. 352.) this was found at Panama in 1560 by a negro who was rewarded with his liberty, and his owner with the office of alcalde of Panama.
Margarita pearls are extremely difficult to find today and are known for their unique yellowish color. The most famous Margarita necklace that any one can see today is the one that then Venezuelan President Romulo Betancourt gave to Jacqueline Kennedy when she and her husband, President John F. Kennedy paid an official visit to Venezuela.
Before the beginning of the 20th century, pearl hunting was the most common way of harvesting pearls. Divers manually pulled oysters from ocean floors and river bottoms and checked them individually for pearls. Not all mussels and oysters produce pearls. In a haul of three tons, only three or four oysters will produce perfect pearls.

 The development of pearl farming

Today, the cultured pearls on the market can be divided into two categories. The first category covers the beaded cultured pearls, including Akoya, South Sea and Tahiti. These pearls are gonad grown, and there is usually one pearl grown at a time. This limits the number of pearls at a harvest period. The pearls are usually harvested after one year for akoya, 2–4 years for Tahitian and South Sea, and 2–7 years for freshwater. This perliculture process was first developed by the British biologist William Saville-Kent who passed the information along to Tatsuhei Mise and Tokichi Nishikawa from Japan. The second category includes the non-beaded freshwater cultured pearls, as the Biwa or Chinese pearls. As they grow in the mantle, where on each wing up to 25 grafts can be implanted, these pearls are much more frequent and do saturate the market completely. An impressive improvement of quality has taken place in the last ten years when the former rice grain-shaped pebbles are compared with the near round pearls of today.
The nucleus bead in a beaded cultured pearl is generally a polished sphere made from freshwater mussel shell. Along with a small piece of mantle tissue from another mollusk (donor shell) to serve as a catalyst for the pearl sac, it is surgically implanted into the gonad (reproductive organ) of a saltwater mollusk. In freshwater perliculture, only the piece of tissue is used in most cases, and is inserted into the fleshy mantle of the host mussel. South Sea and Tahitian pearl oysters, also known as Pinctada maxima and Pinctada margaritifera, which survive the subsequent surgery to remove the finished pearl, are often implanted with a new, larger beads as part of the same procedure and then returned to the water for another 2–3 years of growth.
Despite the common misperception, Mikimoto did not discover the process of pearl culture. The accepted process of pearl culture was developed by the British Biologist William Saville-Kent in Australia and brought to Japan by Tokichi Nishikawa and Tatsuhei Mise. Nishikawa was granted the patent in 1916, and married the daughter of Mikimoto. Mikimoto was able to use Nishikawa's technology. After the patent was granted in 1916, the technology was immediately commercially applied to akoya pearl oysters in Japan in 1916. Mise's brother was the first to produce a commercial crop of pearls in the akoya oyster. Mitsubishi's Baron Iwasaki immediately applied the technology to the south sea pearl oyster in 1917 in the Philippines, and later in Buton, and Palau. Mitsubishi was the first to produce a cultured south sea pearl – although it was not until 1928 that the first small commercial crop of pearls was successfully produced.
The original Japanese cultured pearls, known as akoya pearls, are produced by a species of small pearl oyster, Pinctada fucata martensii, which is no bigger than 6 to 8 cm in size, hence akoya pearls larger than 10 mm in diameter are extremely rare and highly prized. Today, a hybrid mollusk is used in both Japan and China in the production of akoya pearls. It is a cross between the original Japanese species, and the Chinese species Pinctada chemnitzii.

Recent pearl production

China has recently overtaken Japan in akoya pearl production. Japan has all but ceased its production of akoya pearls smaller than 8 mm. Japan maintains its status as a pearl processing center, however, and imports the majority of Chinese akoya pearl production. These pearls are then processed (often simply matched and sorted), relabeled as product of Japan, and exported.
In the past couple of decades, cultured pearls have been produced using larger oysters in the south Pacific and Indian Ocean. The largest pearl oyster is the Pinctada maxima, which is roughly the size of a dinner plate. South Sea pearls are characterized by their large size and warm luster. Sizes up to 14 mm in diameter are not uncommon. South Sea pearls are primarily produced in Australia, Indonesia and the Philippines.
Mitsubishi commenced pearl culture with the south sea pearl oyster in 1916, as soon as the technology patent was commercialized. By 1931 this project was showing signs of success, but was upset by the death of Tatsuhei Mise. Although the project was recommenced after Tatsuhei's death, the project was discontinued at the beginning of WWII before significant productions of pearls were achieved.
After WWII, new south sea pearl projects were commenced in the early 1950s in Burma and Kuri Bay and Port Essington in Australia. Japanese companies were involved in all projects using technicians from the original Mitsubishi south sea pre-war projects.

 Freshwater pearl farming

In 1914, pearl farmers began growing cultured freshwater pearls using the pearl mussels native to Lake Biwa. This lake, the largest and most ancient in Japan, lies near the city of Kyoto. The extensive and successful use of the Biwa Pearl Mussel is reflected in the name Biwa pearls, a phrase which was at one time nearly synonymous with freshwater pearls in general. Since the time of peak production in 1971, when Biwa pearl farmers produced six tons of cultured pearls, pollution has caused the virtual extinction of the industry. Japanese pearl farmers recently cultured a hybrid pearl mussel – a cross between Biwa Pearl Mussels and a closely related species from China, Hyriopsis cumingi, in Lake Kasumigaura. This industry has also nearly ceased production, due to pollution.
Japanese pearl producers also invested in producing cultured pearls with freshwater mussels in the region of Shanghai, China. China has since become the world's largest producer of freshwater pearls, producing more than 1,500 metric tons per year (in addition to metric measurements, Japanese units of measurement such as the kan and momme are sometimes encountered in the pearl industry).
Led by pearl pioneer John Latendresse and his wife Chessy, the United States began farming cultured freshwater pearls in the mid 1960s. National Geographic Magazine introduced the American cultured pearl as a commercial product in their August 1985 issue. The Tennessee pearl farm has emerged as a tourist destination in recent years, but commercial production of freshwater pearls has ceased.

Momme Weight

For many cultured pearl dealers and wholesalers, the preferred weight measure used for loose pearls and pearl strands is momme. Momme is a weight measure used by the Japanese for centuries. Today, momme weight is still the standard unit of measure used by most pearl dealers to communicate with pearl producers and wholesalers. One momme corresponds to 1/1000 kan. Reluctant to give up tradition, in 1891, the Japanese government formalized the kan measure as being exactly 1 kan = 3.75 kilograms or 8.28 pounds. Hence, 1 momme = 3.75 grams or 3750 milligrams.
In the United States, during the 19th and 20th centuries, through trade with Japan in silk cloth the momme became a unit indicating the quality of silk cloth.
Though millimeter size range is typically the first factor in determining a cultured pearl necklace's value, the momme weight of pearl necklace will allow the buyer to quickly determine if the necklace is properly proportioned. This is especially true when comparing the larger south sea and Tahitian pearl necklaces.

Pearls in jewelry

The value of the pearls in jewelry is determined by a combination of the luster, color, size, lack of surface flaw and symmetry that are appropriate for the type of pearl under consideration. Among those attributes, luster is the most important differentiator of pearl quality according to jewelers.
All factors being equal, however, the larger the pearl the more valuable it is. Large, perfectly round pearls are rare and highly valued. Teardrop-shaped pearls are often used in pendants.

Shapes

Pearls come in eight basic shapes: round, semi-round, button, drop, pear, oval, baroque, and circled. Perfectly round pearls are the rarest and most valuable shape. Semi-rounds are also used in necklaces or in pieces where the shape of the pearl can be disguised to look like it is a perfectly round pearl. Button pearls are like a slightly flattened round pearl and can also make a necklace, but are more often used in single pendants or earrings where the back half of the pearl is covered, making it look like a larger, rounder pearl.
Drop and pear shaped pearls are sometimes referred to as teardrop pearls and are most often seen in earrings, pendants, or as a center pearl in a necklace. Baroque pearls have a different appeal; they are often highly irregular with unique and interesting shapes. They are also commonly seen in necklaces. Circled pearls are characterized by concentric ridges, or rings, around the body of the pearl.
In general, cultured pearls are less valuable than natural pearls, whereas imitation pearls almost have no value. One way that jewelers can determine whether a pearl is cultured or natural is to have a gem lab perform an x-ray of the pearl. If the x-ray reveals a nucleus, the pearl is likely a bead-nucleated saltwater pearl. If no nucleus is present, but irregular and small dark inner spots indicating a cavity are visible, combined with concentric rings of organic substance, the pearl is likely a cultured freshwater. Cultured freshwater pearls can often be confused for natural pearls which present as homogeneous pictures which continuously darken toward the surface of the pearl. Natural pearls will often show larger cavities where organic matter has dried out and decomposed.
Some imitation pearls are simply made of mother-of-pearl, coral or conch shell, while others are made from glass and are coated with a solution containing fish scales called essence d'Orient. Although imitation pearls look the part, they do not have the same weight or smoothness as real pearls, and their luster will also dim greatly.

Lengths of pearl necklaces

There is a special vocabulary used to describe the length of pearl necklaces. While most other necklaces are simply referred to by their physical measurement, pearl necklaces are named by how low they hang when worn around the neck. A collar, measuring 10 to 13 inches or 25 to 33 cm in length, sits directly against the throat and does not hang down the neck at all; collars are often made up of multiple strands of pearls. Pearl chokers, measuring 14 to 16 inches or 35 to 41 cm in length, nestle just at the base of the neck. A strand called a princess length, measuring 17 to 19 inches or 43 to 48 cm in length, comes down to or just below the collarbone. A matinee length, measuring 20 to 24 inches or 50 to 60 cm in length, falls just above the breasts. An opera length, measuring 28 to 35 inches or 70 to 90 cm in length, will be long enough to reach the breastbone or sternum of the wearer; and longer still, a pearl rope, measuring more than 45 inches or 115 cm in length, is any length that falls down farther than an opera.
Necklaces can also be classified as uniform, or graduated. In a uniform strand of pearls, all pearls are classified as the same size, but actually fall in a range. A uniform strand of akoya pearls, for example, will measure within 0.5 mm. So a strand will never be 7 mm, but will be 6.5–7 mm. Freshwater pearls, Tahitian pearls, and South Sea pearls all measure to a full millimeter when considered uniform.
A graduated strand of pearls most often has at least 3 mm of differentiation from the ends to the center of the necklace. Popularized in the United States during the 1950s by the GIs bringing strands of cultured akoya pearls home from Japan, a 3.5 momme, 3 mm to 7 mm graduated strand was much more affordable than a uniform strand because most of the pearls were small.

 Colors of pearl jewelry

Earrings and necklaces can also be classified on the grade of the color of the pearl. While white, and more recently black, saltwater pearls are by far the most popular, other color tints can be found on pearls from the oceans. Pink, blue, champagne, green, black and even purple saltwater pearls can be encountered, but to collect enough of these rare colors to form a complete string of the same size and same shade can take years.

 Religious references

 Hindu scriptures

The Hindu tradition describes the sacred Nine Pearls which were first documented in the Garuda Purana, one of the books of the Hindu mythology. Ayurveda contains references to pearl powder as a stimulant of digestion and to treat mental ailments. According to Marco Polo, the kings of Malabar wore a necklace of 108 rubies and 108 precious pearls which was given from one generation of kings to the next. The reason was that every king had to say 108 prayers every morning and every evening. At least until the beginning of the 20th century it was a Hindu custom to present a completely new, undrilled pearl and pierce it during the ceremony.
The Pearl or Mukta in Sanskrit is also associated with many Hindu deities. The most famous being the Koustubha which Lord Vishnu wears on his chest. Apart from religious connotations, stories and folklore abound of pearls occurring in snakes, the Naaga Mani, and elephants, the Gaja Mukta.

 Hebrew scriptures

According to Rebbenu Bachya, the word Yahalom in the verse Exodus 28:18 means "pearl" and was the stone on the Hoshen representing the tribe of Zebulun. This is generally disputed among scholars, particularly since the word in question in most manuscripts is actually Yasepheh - the word from which jasper derives; scholars think that refers to green jasper (the rarest and most prized form in early times) rather than red jasper (the most common form). Yahalom is usually translated by the Septuagint as an "onyx", but sometimes as "beryl" or as "jasper"; onyx only started being mined after the Septuagint was written, so the Septuagint's term "onyx" probably does not mean onyx – onyx is originally an Assyrian word meaning ring, and so could refer to anything used for making rings. Yahalom is similar to a Hebrew word meaning hit hard, so some people think that it means diamond. The variation in possibilities of meaning for this sixth stone in the Hoshen is reflected in different translations of the Bible – the King James Version translates the sixth stone as diamond, the New International Version translates it as emerald, and the Vulgate translates it as jaspis – meaning jasper. There is a wide range of views among traditional sources about which tribe the stone refers to.

New Testament scriptures

In a Christian New Testament parable, Jesus compared the Kingdom of Heaven to a "pearl of great price" in Matthew 13: 45-46. "Again, the kingdom of heaven is like unto a merchant man, seeking goodly pearls: Who, when he had found one pearl of great price, went and sold all that he had, and bought it."
The language of symbolism was in common use around the time of Jesus Christ; most people were familiar with the symbolic meanings. The circle is a symbol of God because it has no beginning and no end. The circle or pearl was considered to represent Love, Knowledge (the combination of equal amounts of Love and Knowledge is a symbol of Wisdom, the 2 circles intertwined (owl eyes) is symbolic of Wisdom. Some other pearls are Truth, and Faith.
The twelve gates of the New Jerusalem are reportedly each made of a single pearl in Revelation 21:21, that is, the Pearly Gates. "And the twelve gates were twelve pearls; every gate was of one pearl: and the streets of the city were pure gold, as if transparent glass."
Holy things are compared to pearls in Matthew 7:6. "Give not that which is holy unto the dogs, neither cast ye your pearls before swine, lest they trample them under their feet, and turn again and rend you."
Pearls are also found in numerous references showing the wickedness and pride of a people, as in Revelation 18:16. "And saying, Alas, alas, that great city, that was clothed in fine linen, and purple, and scarlet, and decked with gold, and precious stones, and pearls!"

Islamic scriptures

The Qur'an often mentions that dwellers of paradise will be adorned with pearls:
22:23 God will admit those who believe and work righteous deeds, to Gardens beneath which rivers flow: they shall be adorned therein with bracelets of gold and pearls; and their garments there will be of silk.
35:33 Gardens of Eternity will they enter: therein will they be adorned with bracelets of gold and pearls; and their garments there will be of silk.
The handsome young boys in paradise are similarly depicted:
52:24 Round about them will serve, [devoted] to them, youths [handsome] as pearls well-guarded.

Other scriptures

The metaphor of a pearl appears in the longer Hymn of the Pearl, a poem respected for its high literary quality, and use of layered theological metaphor, found within one of the texts of Gnosticism.
The Pearl of Great Price is a book of scripture in The Church of Jesus Christ of Latter-day Saints.

Non-religious cultural references

"Pearl" is used as a first name in various languages, usually as a female name. It is also used as family name.
Various locations in different countries are called "pearl" - see Pearl (disambiguation).