A pearl is a hard, round object that is produced by some species of shellfish. It is typically used as a piece of jewelry, and it has a long history of being associated with wealth and luxury. Pearls are formed when an irritant (usually a piece of sand or grit) gets lodged in the shell of the oyster or clam. The shellfish then secretes a substance called nacre, which coats the irritant and eventually forms a pearl. The quality of the pearl depends on the type of shellfish, the size of the irritant, and the conditions in which the pearl is formed. Pearls can be found in a wide range of colors, from white to black, and they are often graded based on their color, luster, and size.