Real Dictionary

mother-of-pearl
noun: a hard iridescent substance, mostly calcium carbonate, that forms the inner layer of the shells of certain molluscs, such as the oyster. It is used to make buttons, inlay furniture, etc
uncountable noun: Mother-of-pearl is the shiny layer on the inside of some shells. It is used to make buttons or to decorate things.

mother-of-pearl in American English
of the mother-of-pearl
the hard, pearly internal layer of certain marine shells, as of the pearl oyster or abalone, which is used in the arts, in making pearl buttons, etc.
adjective: of or having the qualities of mother-of-pearl, as being iridescent or pearly
noun: a hard, iridescent substance that forms the inner layer of certain mollusk shells, used for making buttons, beads, etc.; nacre