Pearls are perhaps the most ancient of gems to be revered by humankind, as they need not be cut or polished as other gemstones do. They are symbolic of purity and wealth, as throughout history they have been prized above all other gems, a status only challeneged by the diamond, and even then only afrer the late nineteenth century when diamonds became more widely available.
A calcareous concretion of calcium carbonate and organic matter secreted by a mollusc in response to an irritation. Layers of nacre and conchiolin are secreted concentrically around a centre or core in response to an irritant. Natural pearls are formed when an object such as a grain of sand gets in between the inner layer of the shell and the mantle of the oyster. Cultured pearls are produced exactly the same way except that the irritant is introduced into the oyster by human means. The beauty of pearls comes from the nacre or irridescent outer layers. The value of pearls is based on colour, lustre, translucency, texture, shape and size. Natural pearls are nacre all the way through, this makes them more durable and more lustrous than a cultured pearl. They are also extremely rare and therefore highly prized.