A mother oyster sitting at the bottom of a warm sea moving its shell just enough to filter plankton from the surrounding water produces a pearl. One day a foreign substance - usually a grain of sand is swept into the shell of oyster and cannot be expelled. In trying to rid itself of irritant, the oyster's body takes defensive action and starts to secrete a smooth hard substance called Nacre that envelops the irritant. Chemically, is composed of very thin crystalline deposits of calcium carbonate creating thousands of thin layers around the irritant. After several years of such an action a pearl is formed.  

Nowadays this process is hastened by pearl farmers who cultivate pearls by inserting a nucleus of shell into the oyster to act as the irritant instead of a grain of sand. The oysters are reared in baskets allowing nature to continue the work of producing the miracle pearl in its own time. This process is akin to the production of a natural pearl except that cultivated pearls are always of superior quality.