The Composition of Mollusk Shells

Periostracum

• The periostracum is the most outside edge of the shell. It is usually lusterless, with a matte finish, because it is weathered by the elements. It's composition is entirely organic conchiolin, according to PearlGuide.com. This outermost layer forms first, while the mollusk is in its glochidaeal, immature stage. The conchiolin, generally brown, black, green or dark blue, is formed by keratin proteins, the same as those found in fingernails and hair.

Ostracum

• The ostracum is the center of the shell between the weathered outer coat and the lustrous inner layer. The ostracum layer is prismatic, and formed entirely of small calcite crystals in a hexagonal shape. According to PearlGuide.com, the crystals measure only one micron. Freshwater mollusks fashion their calcite crystals with aragonite instead, due to minerals found in their locations. The inner shell is yellow and works to provide strength to the shell, serving as a reinforcement to the outer shell, and a layer of defense for the inner shell. It is held together by a thin film and offers a porcelain-like finish.

Hypostracum

• The hypostracum is the mother of pearl layer that is known for its bright colors and prismatic iridescence. This layer grows layer by layer, thicker in some areas and thinner in others, that create detailed high and lows, ridges and edges. The inner shell is comprised almost solely of calcium carbonate. Inside that are innumerable aragonite crystals. These crystals are packed together by conchiolin to hold it all together. It is the conchiolin, tiny aragonite crystals and the mineral traces found in the animal's food source that determines the sheen, color, characteristics, and luster of the inner shell.

  Although some shells don't contain mother of pearl, the Conchologists of America website says that all mollusks make their shells from the same material: calcium carbonate.