Pearls Formed in Seashells

A pearl is a hard object produced within the soft tissue of a living shelled mollusk. A pearl is composed of calcium carbonate in minute crystalline form, which has been deposited in concentric layers.

The ideal pearl is perfectly round and smooth, but many other shapes occur and are known as baroque pearls. The finest quality natural pearls have been highly valued as gemstones and objects of beauty for many centuries.

Pearls are formed inside the shell of certain mollusks as a defense mechanism against a potentially threatening irritant such as a parasite inside the shell, or an attack from outside that injures the inner tissue. The mollusk creates a pearl sac to seal off the irritation.

The mollusk’s mantle (protective membrane) deposits layers of calcium carbonate in the form of the mineral aragonite or a mixture of aragonite and calcite held together by an organic horn-like compound called conchiolin.

The combination of aragonite and conchiolin is called nacre, which makes up mother-of-pearl. The commonly held belief that a grain of sand acts as the irritant is in fact rarely the case.

Typical stimuli include organic material, parasites, or even damage that displaces mantle tissue to another part of the mollusk’s body. These small particles or organisms gain entry when the shell valves are open for feeding or respiration.

The most valuable pearls occur spontaneously in the wild, but are extremely rare. These wild pearls are referred to as natural pearls. Cultured or farmed pearls from pearl oysters and freshwater mussels make up the majority of those currently sold.