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Pearls come in a variety of colors and shades, from white, cream, and pink to black with blue, green, or purple overtones. Many factors are responsible for the color of a pearl, but it is for the most part up to chance. A black-lipped oyster does not usually create a white pearl, and it is very rare for other oysters to produce a black pearl. Other than that, however, there is little certainty about what color pearls you can expect to harvest from certain oysters or certain areas.
The material that makes up a pearl, known as nacre, is secreted by the oyster as a response to an irritant inside the shell. Natural pearls are extremely rare and expensive and consist of concentric layers of nacre around a possibly microscopic original irritant, such as a piece of sand. Cultured pearls are made by introducing a shell nucleus, typically perfectly round, and allowing a few layers of nacre to form around it.
The factors that contribute to the color of a pearl include the type of oyster, the thickness and number of layers of nacre, and possibly trace elements in the oyster's aquatic environment. A pearl manufacturer may also influence the color of the pearl to some degree by introducing tissue from another oyster into the host oyster along with the shell nucleus. The color of a pearl is determined by a combination of factors: the base color, overtone, and iridescence. Cultured pearls are also sometimes dyed.
The thicker the nacre, the richer the color of the pearl will be, and the more iridescence it will show. Thin nacre tends to make for a milky-looking pearl with few overtones. The iridescence of a pearl is due to the overlapping layers of translucent nacre.
The color of a pearl may or may not affect its cost, though uniformity of color makes pearls more valuable. Black pearls used to be quite rare and very valuable, but they are now cultured and consequently much more widely available. Natural white pearls are rarer and more valuable than cultured black pearls. The best color is largely a matter of personal choice, or whatever most flatters one's skin tone, though regional preferences exist and may affect price.