Pearls are formed in saltwater oysters and freshwater clams. A curtainlike tissue called the mantle is within the body of these mollusks. Certain cells on the side of the mantle toward the shell secrete nacre, also known as mother-of-pearl, during a specific stage of the shell-building process.

A pearl is the result of an oyster’s reaction to a foreign body, such as a piece of sand or a parasite, within the oyster’s shell. The oyster neutralizes the invader by secreting thin layers of nacre around the foreign body, eventually building it into a pearl. The thin layers are alternately composed of calcium carbonate, argonite, and conchiolin. Humans can also have a hand in the formation of pearls—in particular, they intentionally place irritants within an oyster—resulting in what we often see in a jewelry display as “cultured pearls.”