What is a pearl and how is it made?

A pearl is a hard, round object produced within the soft tissue of a living mollusk, typically an oyster or a mussel. It forms as a defense mechanism against irritants, such as a grain of sand, when the mollusk secretes layers of nacre, a substance made of calcium carbonate and conchiolin. Over time, these layers accumulate, resulting in the formation of a pearl. Natural pearls are rare, while cultured pearls are produced through human intervention by deliberately introducing an irritant into the mollusk.