Pearl

Pearl, hard, rounded gem produced by certain bivalve mollusks, particularly pearl oysters (Pinctada) and the freshwater pearl mussel (Margaritifera margaritifera). In response to an irritation by foreign matter within the shell, the mantle secretes calcium carbonate in the form of nacre (mother-of-pearl) around the irritant body. Over several years, this encrustation forms the pearl. Cultured pearls may be obtained by “seeding” the oyster with an artificial irritant such as a small bead. Pearls are variable in shape and may be black or pink as well as the usual white.

See also: Oyster.