Pearls rarely start with a grain of sand
The familiar story says a pearl begins when a grain of sand slips into an oyster. That is rarely true. The usual trigger is a parasite, a scrap of organic matter, or an injury that pushes shell-making cells into the wrong place, where they keep doing their job and slowly coat a tiny sac in mineral.
A pearl is built from calcium carbonate, mostly the mineral aragonite, laid down in thin concentric layers by the mollusc's mantle and bound with a horn-like protein called conchiolin. That combination is nacre, the same material as the mother-of-pearl lining the shell. Almost any shelled mollusc can make some kind of pearl, but gem-quality ones come mainly from saltwater pearl oysters and freshwater mussels. Because they are carbonate, pearls dissolve in vinegar, releasing carbon dioxide.
Growth can be traced in cross-section. When a predator, crab or worm larva damages the fragile shell rim, some outer mantle cells can be torn loose and embedded deeper in the tissue. They survive, form a pocket called a pearl sac and go on secreting carbonate: first a brown core of columnar crystals, then a whitish coat of nacre. The sac stays put while the shell grows around it, and after a couple of years a pearl may be waiting for a lucky fisher.
Shine is an optical effect. Light reflecting, refracting and diffracting through the translucent layers produces lustre, finer when the layers are thinner and more numerous, and the overlapping layers create iridescence. The most prized pearls are perfectly round with a metallic sheen; irregular ones are called baroque.
Wild pearls are extraordinarily rare, since hundreds of oysters or mussels must be opened, and killed, to find one, which is why they once commanded astonishing prices. Today most pearls are cultured by inserting a piece of mantle tissue, often with a bead, and most freshwater cultured pearls come from China. The English word traces back to Latin perna, 'leg', after a ham-shaped shellfish, and the Old Persian word for pearl is the root of the name Margaret.
Source: Pearl
