Scallops normally are found in shallow waters of protected bays with sandy or muddy bottoms. "Scallop" is the common name of a group of about 360 species of edible bivalve mollusks. Mollusk shells consist of a calciferous substance that is secreted by the soft mantle tissue and then hardens. Shells have three layers: an inner, shiny layer of nacre, or mother-of-pearl, which is a mixture of calcium carbonate and the protein conchiolin; a central, prismatic layer of calcium carbonate crystals; and an outer, organic layer, the periostracium, made of conchiolin. Many mollusk shells are important commercially because of their beauty and rarity. Shells of the more common mollusks long have been used for the production of synthetic pearls, pearl buttons, and other decorative items.