I know it’s in a movie title, but is there such a thing as a black pearl? Are there other colors?
Nonwhite pearls exist, but to explain them it’s best to start with a little background.
Only a few species make beautiful pearls, but pretty much any mollusk with a shell can form a pearl. The process is a response to a small, irritating object being caught inside the shell. The animal coats the object with layers of nacre, or mother-of-pearl, the material that forms the inside of its shell.
The outer layers of nacre are composed of hexagonal plates about 10 to 20 millionths of a meter wide and about half a millionth of a meter thick, all embedded in organic polymers like chitin (which forms the shells of insects and crabs) and various silk-like proteins.
The iridescence of nacre comes from the complicated scattering of light as it bounces off the tiny crystals. Modern technology has a hard time reproducing the properties of nacre, so fake pearls tend to be unconvincing.
Clams are particularly prone to forming pearls in saltwater, while mussels do it more frequently in fresh water. Getting wild pearls is hit and miss, since you have to find a shell that has formed one. Cultured pearls are grown by cultivating the mollusks and putting bits of shell into them. The two types of pearls are quite different, with natural pearls usually formed from many layers, and cultured pearls typically made from one layer.
Black pearls are made by a variety of the giant black-lipped oyster Pinctada margaritifera. These oysters produce a black-tinted nacre, so the pearls formed wind up gray to black.
Black pearls are rare, and it is difficult to farm them. Single, high-quality pearls can cost more than $1,000. As for other colors, the Caribbean pink conch can produce pink pearls. South Sea pearls can be white, silver, cream, pink, gold, or a mixture.
Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to drknowledge@ globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.