A pearl is a hard, round object produced by certain mollusks, primarily oysters and clams. It forms when an irritant, such as a grain of sand, enters the shell. The mollusk secretes layers of a substance called nacre around the irritant, eventually creating a pearl. Pearls can vary in size, shape, and color, depending on the species of mollusk and environmental factors. Natural pearls are rare, leading to the development of cultured pearls, which are created by intentionally introducing an irritant into a mollusk. Pearls are valued in jewelry and are often associated with elegance and beauty, making them popular in accessories and fashion.