mother-of-pearl or nacre, the iridescent substance that forms the lining of the shells of some fresh-water and some salt-water mollusks. Like the pearl it is a secretion of the mantle, composed of alternate layers of calcium carbonate and conchiolin. Among the chief sources are the pearl oyster, found in warm and tropical seas, chiefly in Asia; freshwater pearl mussels, which live in many rivers of the United States, Europe, and Asia; and the abalone of California, Japan, and other Pacific regions.