Sensing the object the living organism within the mollusk secretes calcium carbonate, a derivative mineral of aragonite, and the binding protein conchiolin. This nacre, Mother Of Pearl, is the same substance 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 calcium crystals together. These substances combine and the regularity of the nacre (Mother Of Pearl) layers results in giving the Pearl what is called its luster.