<Li> Yellow and golden pearls: yellow - green, greenish brown to dark brown; </Li> <Li> Black pearls: commonly pink to orange - red </Li> <P> A pearl is a hard glistening object produced within the soft tissue (specifically the mantle) of a living shelled mollusk or another animal, such as a conulariid . Just like the shell of a mollusk, a pearl is composed of calcium carbonate (mainly aragonite or a mixture of aragonite and calcite) in minute crystalline form, which has been deposited in concentric layers . The ideal pearl is perfectly round and smooth, but many other shapes, known as baroque pearls, can occur . The finest quality natural pearls have been highly valued as gemstones and objects of beauty for many centuries . Because of this, pearl has become a metaphor for something rare, fine, admirable and valuable . </P> <P> The most valuable pearls occur spontaneously in the wild, but are extremely rare . These wild pearls are referred to as natural pearls . Cultured or farmed pearls from pearl oysters and freshwater mussels make up the majority of those currently sold . Imitation pearls are also widely sold in inexpensive jewelry, but the quality of their iridescence is usually very poor and is easily distinguished from that of genuine pearls . Pearls have been harvested and cultivated primarily for use in jewelry, but in the past were also used to adorn clothing . They have also been crushed and used in cosmetics, medicines and paint formulations . </P>

Which gem stone is produced by the secretion of some sea creatures