Answer: Nacre

Nacre, commonly known as mother of pearl, is the iridescent layer found on the inner surface of some mollusc species. The composite compound is made from a combination of organic and inorganic material and, on a microscopic level, is composed of tiny little hexagonal platelets of aragonite (a form of calcium carbonate) locked in place by layers of elastic biopolymers (such as chitin, lustrin and silk-like proteins).

While the compound appears in several different mollusc groups, of most interest to us (and to the jewelry industry) are the pearl oyster and the freshwater pearl mussel; molluscs that not only produce the lining along the inside of their shell, but also produce balls of the lining. Pearls are simply thousands upon thousands of layers of nacre produced and layered away from the inner surface of the shell.