Nacre is an organic-inorganic mineral secreted by the epithelial cells found in the mantle tissue of a mollusk which both coats the inside of the shell and also makes up the crystalline structure of a pearl. Nacre is composed of calcium carbonate (CaCO3) and aragonite crystalline platelets, interspersed with organic glue, also known as conchiolin.
The mollusk continuously layers both the inside of its shell as well as any irritants trapped inside with this material to form both a hard, protective covering for its soft body as well as to smooth over any irritants trapped inside. The resulting structure resembles an onion or a brick wall with alternating layers of aragonite and conchiolin creating an incredibly strong, flexible and beautiful matrix. Nacre gets its iridescence from light striking and penetrating the surface of the semi-transparent aragonite crystals, and absorbing and refracting certain wavelengths of the light spectrum.
See also: Pearls, Calcium Carbonate, Cultured Pearls, Natural Pearls