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What Causes a Black Pearl to Form?

Pearls, like coral and amber, are one of the few gemstones created by a living organism. Geological processes, such as the extreme pressure found deep underground, produce most other gemstones, such as rubies and diamonds. Mollusks produce not only pearls, but also mother of pearl which is cut from their shells.

Pearl Formation
- Oysters and mussels create pearls as a form of defense against particles that have worked their way inside the mollusk's shell. If the foreign particle finds its way to a spot that irritates the oyster or mussel, it secretes a substance called nacre which surrounds the particle creating a smooth, non-irritating surface. This process is very slow requiring the deposit of thousands of thin layers around the particle; it takes up to seven years to grow a pearl.
Nacre
- Pearls and the shell of the oyster are composed of nacre. No matter which species of bivalve creates the pearl, the composition of the nacre is about the same. Calcium carbonate, otherwise called carbonate of lime, makes up the bulk of the pearl's composition, over 90 percent. Throw in six percent of conchiolin, an organic material, and a little water and you have a pearl.
Natural or Manmade
- Natural pearls have always been very rare. Only two to three percent of oysters form pearls in nature. But their unusual beauty created a lot of demand for pearls. Almost all pearls on the market today are cultured pearls. Placing a seed into a fleshy area inside the oyster starts pearl formation. Pearl farmers make the seed from a piece of freshwater clam shell. Not only oysters make pearls. Freshwater mussels also form pearls, but these pearls are not round like salt-water pearls. Instead, the freshwater bivalve species form irregularly shaped, oblong pearls.
Pearl Characteristics
- Jewelers grade pearls on their shape, size, luster and color. The most valuable shape for a pearl is perfectly round with no indentations or markings. The value of the pearl varies with its size as well. Very small pearls measure 3 millimeters in size, medium pearls measure between 5 and 6 millimeters in size and very large pearls measure over 8 millimeters. Luster is more difficult to quantify, but comprises a glow created by many layers of semi-transparent nacre. A pearl with poor quality, thin nacre looks chalky rather than luminescent. Finally, jewelers grade pearls by color. Americans favor deep pink colored pearls most, followed by white, then cream although black colored pearls are popular as well.
Black Pearls
- Only one species of oyster native to the South Pacific grows natural black pearls. The Pinctada margaritifera, or black-lip pearl oyster, creates lustrous silver to black pearls. Because of the high demand for their mother of pearl shells, as well as their black pearls, overfishing nearly drove them to extinction.
  
  The black-lip pearl oyster grows quite large -- the size of a dinner plate -- and can produce several pearls at once. The black color of the pearl comes from the color of the nacre exuded by the black-lip pearl oyster.

Nature