The outer skeleton or shell of a mollusk is composed of thin sheets of conchiolin impregnated with crystals of calcium carbonate and small quantities of calcium phosphate and magnesium salts. The inner surface of the shell in some mollusks, such as pearl oysters, river mussels, and abalones, is an iridescent pearly layer. The iridescence is caused by the interference of light reflected from the layers of mother-of-pearl. These layers consist of thin, overlapping plates of calcium carbonate deposited over sheets of conchiolin.

The shapes assumed by mollusk shells are highly variable. In relatively primitive forms called chitons, the mantle produces eight transverse calcareous plates on the upper surface of the animal. In snails, the shell is a coiled structure. The snail mantle begins as a small structure produced posterior to a prominence called a visceral hump. During one of the early stages of development, the hump rotates through a sweep of 180 degrees. The twisting undergone during this rotation gives the mantle a spiral shape, and so the layers of shell material subsequently produced by the mantle also have a spiral shape. Several species of bivalve, notably the pearl oyster and freshwater mussels, produce pearls often used as jewels, and the shells of many bivalves are sources of mother of pearl.