Pearls are formed by many mollusks as part of a natural process to lessen the effect of irritating debris. When a small piece of particulate matter becomes trapped inside the mollusk, calcium carbonate is deposited on its surface, and over time this calcium carbonate (called nacre) forms a pearl. Black pearls are made by a species of oyster called Pinctada margaritifera, commonly known as the black-lipped oyster.
Many bivalves generate pearls, and most are desirable for jewelry. Of the mollusks which create pearls, including abalone and mussels, the pearls created by oysters are the most sought after. These pearls may come in a variety of different colors and degrees of quality, depending on the species of oyster, the seed object, and a number of other variables.
The Pinctada margaritifera oyster appears in the South Pacific. Historically, most black pearls came from Tahiti, and they are therefore known as Tahitian black pearls. In recent years, however, both nearby Kiribati and the Cook Islands have begun producing these pearls, accounting for 3-4% of the world's supply.
In the past, black pearls were amazingly expensive due to their extreme rarity. Approximately one in ten-thousand oysters produces a pearl that is black, and of these, a small fraction are of adequate luster, shape and size to be desirable. They are, therefore, associated with high luxury and class in jewelry, and many prestigious necklaces and bracelets of royalty and the elite contain ones of large size.
Since the 1960s, black pearls have been cultured, causing a dramatic decrease in price. A piece of Mississippi freshwater clam shell is used as a nucleus, placed near the oyster's genitals, along with a bit of flesh from the oyster to serve as a mantle. Over approximately the next two years, the oyster places thousands of layers of nacre and ultimately creates a pearl.
These pearls are substantially larger on average than their more common white brethren. Most white (and off-white) pearls come from the Japanese Akoya oyster, which rarely exceeds 3 inches (7.6cm) in diameter. Pinctada margaritifera can exceed 1 foot (30.4 cm) in diameter, allowing for pearls in excess of 0.5 in (12 mm).
Black pearls, it should be noted, are rarely actually black in color. They range from nearly black to white, and many are silver, but the rarest and most valuable are a deep purplish green. The luster of a pearl is also a major factor in its apparent color, with the best pearls having a translucent sheen that reflects a number of different colors, depending on the angle from which it is viewed.
Although harvesting natural oysters for pearls in places like Tahiti is illegal, with the advent of cultured black pearls, prices have become more affordable for these rare beauties. Even the smallest of such pearls are certain to draw attention and praise from all who see them, and they are often a wonderful addition to a special piece of jewelry.