Source : Webster's Revised Unabridged Dictionary (1913)

Mother-of-pearl \Moth"er-of-pearl`\, n. (Zo["o]l.)
   The hard pearly internal layer of several kinds of shells,
   esp. of pearl oysters, river mussels, and the abalone shells;
   nacre. See {Pearl}.

Source : WordNet®

mother-of-pearl
     n : the iridescent internal layer of a mollusk shell [syn: {nacre}]