Calcium carbonate is a primary component of the shells of marine mollusks and pearls. The outer shells of mollusks are composed of calcium crystals bound by conchiolin. The middle layer is formed by calcium carbonate hardened as crystalline calcite. The inner layer, mother-of-pearl, is made up of nacre – a calcium carbonate that crystallizes as aragonite (orthorhombic) and is bound by conchiolin. The nacreous coating on pearls is also composed of nacre (calcium carbonate crystallized as aragonite).