A natural pearl is formed by a mollusk - an animal such as an oyster, clam, conch, or gastropod.
How Does a Pearl Form?
Pearls are formed when an irritant, such as a bit of food, a grain of sand, or even a piece of the mollusk's mantle becomes trapped in the mollusk. To protect itself, the mollusk secretes substances that it also uses to build its shell - aragonite (a mineral) and conchiolin (a protein). These substances are secreted in layers and a pearl is formed.
Depending on how the aragonite is formed, the pearl may have a high luster (nacre, or mother-of-pearl) or a more porcelain-like surface.
A wild pearl often has imperfections. One way to tell a natural pearl from an artificial pearl, according to the American Museum of Natural History, is to rub it against your teeth. A natural pearl will feel gritty, and an artificial pearl will feel smooth.
Pearls created in the wild are rare and expensive. Eventually, people began culturing pearls, which involves placing an irritant in the shells of mollusks. They are then placed in holding baskets and the pearl is harvested after about 2 years.
Species That Form Pearls
Any mollusk can form a pearl, although they are more common in some animals than in others. There are animals known as pearl oysters, which includes species in the genus Pinctada. The species Pinctada maxima (called the gold-lipped pearl oyster or silver-lipped pearl oyster) lives in the Indian Ocean and the Pacific from Japan to Australia and produce pearls known as South Sea Pearls. Other pearl-producing animals include abalones, conchs, pen shells, and whelks. Pearls may also be found and cultured in freshwater mollusks and are often produced by species collectively called "pearl mussels."