Nacre ( /ˈneɪkər/ NAY-kər), also known as mother of pearl, is an organic-inorganic composite material produced by some molluscs as an inner shell layer; it is also what makes up the outer coating of pearls. It is very strong, resilient, and iridescent.
Nacre is found in some of the more ancient lineages of bivalves, gastropods and cephalopods. However, the inner layer in the great majority of mollusc shells is porcellaneous, not nacreous, and this usually results in a non-iridescent shine, or more rarely in non-nacreous iridescence such as flame structure as is found in conch pearls.
The outer layer of pearls and the inside layer of pearl oyster and freshwater pearl mussel shells are made of nacre. Many other families of Mollusc also have a nacreous inner shell layer, including marine gastropods such as the Haliotidae, the Trochidae and the Turbinidae.
Read more about Nacre: Evolution, Commercial Sources, Decorative Uses, Artificially Created Nacre, Value