All waterborne organisms living within shells, called mollusks, have the ability to produce Mother of Pearl . However, very few mollusks with single shells, called gastropods, do so. High quality Mother of Pearl is produced by the mollusk family called bivalves, which have two-part shells.
The popular belief that Mother of Pearl and Pearl is formed as a result of a grain of sand entering a mollusks shell is false. Mother of Pearl or Nacre forms when an organic irritant such as a parasite, food-particle become trapped within the mollusk, or if the mollusk is injured in some way. Sensing the object the living organism within the mollusk secretes calcium carbonate, a derivative mineral of aragonite, and the binding protein conchiolin, these are the same substances that the animal uses to build its shell. The layers of calcium carbonate settle on the intruder and are interspersed by the conchiolin, which acts as a kind of organic glue binding the crystals together. The regularity of the crystals, and the number of layers results in giving the Mother of Pearl what is called luster.