Pearls are formed in shellfish - especially oysters and mussels - as a natural defense against an irritant, such as a piece of grit. Layers of aragonite, known as nacre, are secreted around the irritant, and gradually build up to form the solid pearl. Light reflecting from these overlapping layers produces a characteristic iridescent luster, also known as the "orient of pearl". In cultured pearls an irritant is introduced to initiate the formation of a pearl. In a "nucleated" cultured pearl a small bead is used as the nucleus, upon which the layers of nacre are secreted. Pearls vary in color from white, white with a hint of color (often pink), to brown or black, depending on the type of mollusc and the water. They are sensitive to acids, dryness, and humidity, and so are less durable than many other gems.

Natural pearls have been harvested from the Persian Gulf, the Gulf of Manaar (Indian Ocean), and the Red Sea for thousands of years. The coasts of Polynesia and Australia produce mainly cultured pearls. Both freshwater and saltwater pearls are cultivated in Japan and China. Freshwater pearls occur in the rivers of Scotland. Ireland, France. Austria. Germany and Mississippi (USA).

Pearls were once thought to be the tears of the gods.