Pearls are beautiful, lustrous gems formed inside the shells of certain mollusks, primarily oysters and clams. They are created 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 forming a pearl. Pearls can vary in size, shape, and color, with the most common being white or cream. There are two main types of pearls: natural pearls, which occur without human intervention, and cultured pearls, which are produced with human assistance. Cultured pearls are more common and are often farmed in controlled environments. Pearls have been valued for centuries and are used in jewelry and decorative items.