Saltwater pearls occur mostly along the coast of India, in the Persian Gulf, and in the Red Sea, and are produced by oysters. Freshwater pearls occur in lakes and rivers and are produced by mussels. Oysters can produce only one pearl at a time, while mussels can produce many.
The chemical formulas of conchiolin and calcium carbonate are C30H48O11N9 and CaCO3, respectively. Combined, they are an organic/inorganic compound known as nacre, or mother-of-pearl.
Nacre is 95% calcium carbonate. Calcium carbonate exists within pearls as hexagonal plates roughly half a micrometer thick, a crystalline formation known as aragonite. Nacre is iridescent because the thickness of aragonite crystals is close to the wavelength of visible light.
A brittle ceramic on its own, aragonite lays between fibrous layers of conchiolin. Calcium carbonate assumes the form of aragonite rather than calcite, which represents the bulk of sea shells, because conchiolin is ionic. This chemical-structural interaction generates impressive strength and is distantly similar to the curing of man-made epoxies. Fossil nautiloid shells unearthed in Oklahoma continue to diffract light.
Concholin is secreted along with other proteins from the mantles of various mollusks. Concholin is chamber-shaped, enclosing and bonding with argonite. Functionally, nacre is a kind of permanent mucus; the vast majority of this world's natural pearls are thinly-veneered and expelled shell fragments. The sizeable, round hanks of nacre occasionally produced by oysters are the results of years of uniform irritation. An x-rayed pearl looks like a tree's growth rings.
Despite its relative strength, nacre has experienced almost no functional relationship with humanity.
A pearl dissolved in vinegar is allegedly the most expensive meal in human history, worth eight figures in 2014 US Dollars. This, because of an impromptu wager between Cleopatra and Mark Antony to, well, consume the most expensive meal possible. We're told that Mark Antony could not bring himself to drink. To unfurl every subtext of that act is perhaps beyond my ken, but I can tell you Egypt was Rome's client state in those days, and that Cleo herself stood on an inbred, comparatively incompetent royal bloodline.
Pearls' value in the ancient world is hard to overstate. All cultures with coastline access treasured them obsessively. They were inconceivably rare, even near the Persian Gulf's natural oyster beds. The Koran places pearls among the most desirable objects in Paradise; Ancient Greeks, with their thousand folded miles of beach, displayed pearls at weddings, believing them to absorb love. Documentation of humans' obsession with pearls goes back 4000 years.
In the Hindu religion an appropriate wedding gift is a pearl, undrilled, along with its piercing. As with many attractive and fairly uniform substances, pearls symbolize purity. It was Krishna who discovered the first pearl. Pearls bear much emotional baggage indeed.
Pre-colonial America, North and South, enjoyed a relative abundance of freshwater pearls, and as elsewhere, they were ornamentation for weddings in particular. They were more or less gone from the hemisphere by mid-nineteenth century, being one of the first substances siphoned to Europe.
The culturing of pearls seems a shockingly recent development when one considers everything that goes into gardening, foie gras, and beer. The first cultured pearls appeared in Japan at the turn of the 20th century. Spouses Kokichi and Ume Mikimoto, and biologist Tokichi Nishikawa and carpenter Tatsuhei Mise, each wrapped a grain of sand in oyster epithelial membrane and put the assembly into an oyster.
The Nishikawa/Mise pair won the patent first. The Mise-Nishikawa Method continues to be synonymous with the culturing of pearls. Because Mise took the extra step of patenting a grafting needle, the Mikimiotos were unable to use the Mise-Nishikawa method without invalidating their own patents (they had patented the use of epithelial tissue wrapped around an irritant only). In 1916, the spouses seized upon a technicality, patenting a method to produce round pearls, and Mise and Nishikawa fell into obscurity. Kokichi Mikimoto thenceforth enjoyed a reputation as showman and pest to gem firms and governments.
Today's cultured saltwater pearls are nucleated with, oddly enough, shell bands from American mussels. This was Kokichi's discovery, a result of much trial-and-error. The culturing and inclusion of pearls in jewelry still requires much; thousands of pearls must be sorted through for a single necklace, and drilling requires a machinist's precision and finesse.
Oysters are nucleated at three years of age and hang from rafts suspended in the ocean. Time spent hanging, between one and three years, is determined by the size of pearl desired. Harvest is in Winter. Cultured saltwater pearls are mostly mussel shell with a thin layer of nacre. Freshwater pearls, meanwhile, are all nacre (tissue only is used,) and are more easily induced. Round pearls are the obvious preference in any case, but different shapes can be built around different nuclei.
A pearl's color is vulnerable to water temperature, diet, and mollusc breed. Even today, hatcheries cannot control or predict the colors of finished pearls. So-called "black pearls" are usually very dark blue or green; natural actually-black pearls are known to occurr around French Polynesia.
Not sure if a pearl is real? Rub it against your teeth. The aragonite crystals, smooth to your fingertips, will grate against your tooth enamel.
All About Gemstones. "Natural & Artificial Pearls: Composition & Chemistry."
Human Touch of Chemistry. "How do Oysters Make Pearls?"
Pearl. "Composition of Pearl."
Fred Ward, pbs.org. "The History of Pearls."
American Pearl. "A Brief History of Pearls."
Pearl Oasis. "Pearl History."
Pearl Guide. "The History of Pearls."
How Products are Made. "Cultured Pearl."