Pearls are formed inside mollusks such as oysters and mussels, taking up to seven or eight years to be created. They are formed when an irritant such as a tiny stone or bit of sand gets inside the mollusk's shell. A lustrous substance, called nacre, is secreted around the object to protect the soft internal surface of the mollusk. As layer upon layer of nacre coats the irritant, a pearl is formed. Each pearl coloration will depend on the type of mollusk and the water where the mollusk lived. Light that is reflected from these overlapping layers produces a characteristic iridescent luster.

Pearls vary in color from white to those with a hint of color, often pink, to brown or black.

Natural pearls have been harvested from the Persian Gulf, the 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 the United States.