Mother-of-pearl

from WordNet (r) 3.0 (2006)

mother-of-pearl
    n 1: the iridescent internal layer of a mollusk shell [syn:
         {mother-of-pearl}, {nacre}]

from The Collaborative International Dictionary of English v.0.48

Mother-of-pearl \Moth"er-of-pearl`\, n. (Zool.)
   The hard, iridescent, pearly internal layer of several kinds
   of shells, esp. of pearl oysters, river mussels, and the
   abalone shells; nacre. See {Pearl}.
   [1913 Webster]