A natural pearl is formed by deposits of nacre around an irritant which accidentally lodges within the body of an oyster. The deeper the luster and iridescence, the more precious the pearl. The more symmetrical the shape the more valuable. Satiny smooth surfaces are desirable. Pearls of irregular and asymmetrical shape are termed baroque.
Oysters grow pearls in many different shapes, from perfect spheres to long, flat angel wing pearls.
The Akoya pearl is the most familiar type of cultured pearl, grown in true pearl oysters off the coasts of Japan. They are known for their lovely orient and warm color. Usually silvery in appearance, and sometimes not as lustrous as fine Akoya pearls, South Sea cultured pearls are rare and costly. Similar to South Sea pearls, these are grown off the coast of Burma and are warmer in color tone than the South Sea pearls. The Burmese pearls are also rare and costly.