Natural or real pearls are made by oysters and other mollusks. Cultured pearls also are grown by mollusks, but with human intervention; that is, an irritant introduced into the shells causes a pearl to grow. Imitation pearls are man-made with glass, plastic, or organic materials.
Because natural pearls are very rare, most pearls used in jewelry are either cultured or imitation pearls. Cultured pearls, because they are made by oysters or mollusks, usually are more expensive than imitation pearls. A cultured pearl's value is largely based on its size, usually stated in millimeters, and the quality of its nacre coating, which gives it luster.