Nacre, also known as mother-of-pearl, is the basic substance which is secreted by oysters and mollusks to form the inside of their shells. When nacre secretions are deposited around a foreign substance which has invaded the mollusk's body, they build up to form a pearl.
Nacre is composed of layers of calcium carbonate (in a crystalline form) and conchiolin (an organic protein substance which provides bonding). The specific luster, iridescence, and coloring of nacre -- and, therefore, of any pearl which it forms -- depends on the number and thickness of the various layers, as well as on whether or not the layers overlap one another.
