Pearls are made by oysters and other mollusks such as mussels. When a foreign object, such as a grain of sand, enters the oyster's shell, the oyster secretes a substance called nacre to coat the irritant and protect its soft tissue. Over time, layer upon layer of nacre is deposited on the irritant, forming a pearl. Pearls come in many shapes and sizes and are valued for their beauty and rarity. They have been used in jewelry for centuries and are associated with luxury and elegance.