Calcium carbonate (CaCO3) is the crystalline mineral that makes up the shells of oysters and freshwater mussels; it is also commonly found in rocks, egg shells, snail shells and pearls. A key ingredient in pearl nacre, calcium carbonate is the white crystalline aragonite layer which is layered concentrically with organic conchiolin, which is a brownish organic "glue" that holds all the layers together.
See Also: Aragonite, Pearl, Nacre