A pearl's color is determined by that of the conchiolin, which is the dark-colored substance released by the shell lining during the early stages of pearl formation. Historically, many theories were put forward to explain how pearls attained their color. At one time it was thought that dark pearls were formed in cloudy weather and white pearls were formed when the weather was fair.
Tahitian pearls are found in the French Polynesian Islands. Tahitian pearls are always described as "black," although they can also be various colors. They are more expensive than white pearls because the black-lipped oysters that produces them are rarer and require more delicate farming. It takes two years to cultivate them. These saltwater oysters produce an array of iridescent black, gray and greenish-black pearls. The oyster's unusually thick nacre, which is the iridescent internal layer of the oyster's shell, produces these vivid colors. The cultivation and export of authentic Tahitian pearls is government-controlled, to ensure quality and minimum standards.
Historically known as "New World pearls," Mexican pearls are prized because of their beautiful luster. Their color spectrum ranges from opalescent white to bronze, blue, pinkish-violet and jet black. By the 16th century the “black pearls” of the Gulf of California were known as the "Queen of Gems" because so many of them could be found nestling in the crowns of European kings and queens. In Europe, the Americas were known as “the land where pearls come hither.”
Greenish-black pearls are highly valued because of their rarity. Good quality blue-black pearls are also prized and command almost the same price as a black pearl. Naturally formed pearls are rare. Over 99.9 per cent of the pearls found on the market today are cultured.
Many pearls labeled as" black pearls" have been artificially dyed. Be wary of buying black pearls selling for a couple of hundred dollars. A quality strand of black pearls would set you back thousands of dollars. You can check the authenticity of black pearls by having a jeweler examine them.
Pearls were reserved for royalty and the very rich before the onset of pearl culturing in the early 1900s. The historian Suetonius wrote that the Roman general Vitellius financed an entire military campaign by selling just one of his mother's pearl earrings.