You know this one. It is formed on the inside of the shells of molluscs. Sometimes called nacre, it is composed of hexagonal platelets of aragonite (calcium carbonate crystals) 10-20 µm wide and 0.5 µm thick, arranged in parallel layers. These layers are separated by sheets of an organic matrix composed of elastic biopolymers (such as chitin, lustrin and silk-like proteins). This mixture of brittle platelets and the thin layers of elastic biopolymers makes the material strong and resilient and creates the lustre effect for which it is prized.
All waterborne organisms living within shells, called molluscs, have the ability to produce Mother of Pearl. However, very few molluscs with single shells, called gastropods, do so. High quality Mother of Pearl is produced by the mollusc family called bivalves, which have two-part shells. Oysters and mussels to us common folk.