Nacre

n. mother-of-pearl, shiny transparent substance that lines many shells

Nacre, also known as mother of pearl, is an organic-inorganic composite material produced by some mollusks. It is strong and resilient, and appears irridescent.  Pearls and the inside layers of an oyster shell are made of nacre.
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Noun
1. the iridescent internal layer of a mollusk shell
(synonym) mother-of-pearl
(hypernym) shell

n. mother of pearl, iridescent lining of a mollusk shell (used for decorative purposes)

adj. nacreous, pearly

(n.)
A pearly substance which lines the interior of many shells, and is most perfect in the mother-of-pearl. [Written also nacker and naker.] See Pearl, and Mother-of-pearl.