An oyster can produce a pearl when an irritant becomes trapped inside its soft tissues and is coated with layers of nacre.
Nacre is a hard, shimmering material made mainly from calcium carbonate and organic compounds. The oyster deposits it around the irritant over time, creating the characteristic layered structure of a pearl. In nature, the trigger is not usually a deliberately inserted grain of sand.
Pearl oysters belong mainly to the genus Pinctada and are marine bivalves. Freshwater mussels can also produce pearls, so the common phrase “oyster pearl” is more specific than the broader word “pearl.” Cultured pearls are formed with human assistance, but the oyster still produces the nacre.
Oysters are also important filter feeders and reef builders in coastal ecosystems. Their shells provide habitat for other organisms, while their filtering can improve water clarity.