A cultured pearl is a pearl formed by an oyster, composed of concentric layers of a crystalline substance called nacre deposited around an irritant placed in the oyster's body by man. Natural pearls are formed by deposits of nacre around an irritant which accidently lodges within the body of an oyster. The value of a pearl comes from the unique ability of the crystalline nacre to absorb, refract and reflect light, imparting a distinctive quality, termed orient, to natural and cultured pearls. Orient is the deep inner glow and shimmering irridescent characteristic of sea-grown pearls. The deeper the lustre and irridescence, the more precious the pearl.
Size contributes to the price of a pearl.
As it is more difficult for oysters to grow large pearls, they are scarcer. But two pearls of different sizes may be valued the same if the smaller pearl is superior in orient to the larger. Shape determines value, too. The more symmetrical the shape, the more valuable. Pearls of irregular and asymmetrical shape are termed baroque.