A pearl is a hardened 'stone' formed inside certain types of oysters as a protection against grit that has entered their shells. Black pearls are not actually black, but very dark gray, with overtones such as blue, green or purple. For centuries, they were prized for their rarity; most pearls are white, cream-colored or a very pale shade such as pink. White pearls can be stained to appear to be naturally black by dipping them in a solution of silver nitrate. Experts can easily distinguish a phony black pearl from the real thing.
However, real black pearls are becoming increasingly common as cultured, or cultivated, pearls, and their price correspondingly has become much more affordable. Black pearls can vary widely in darkness of color, hue and iridescence. Iridescence is that quality which allows a stone to show different colors when light strikes from different angles.
Culturing pearls is a practice over a hundred years old. To 'grow' pearls, rather than harvest oysters and look for naturally occurring pearls, a tiny bit of grit is introduced into the shell of an oyster. Over two or three years, the oyster secretes a coating over the grit to prevent it from irritating the oyster's soft flesh. The coating is the same shade as the nacre or mother-of-pearl lining of the oyster's natural shell. One species of oyster native to the Polynesian islands, the Pinctada margaritifera or giant blacklipped oyster, will produce pearls of a dark color, or 'black' pearls, when cultured.
Black pearls are quite popular in modern jewelry-making. They are either matched in color and iridescence, or black pearls of different undertones and hues are strung together. Sets featuring perfectly matched pearls are much more costly, due to the difficulty in finding stones that are indistinguishable from one another.