I saw several 'chocolate' strands at the show - very expensive. I have not quite figured out why they are so popular. They are basically color-enhanced Tahitian pearls. The cheapest I saw was $7000 for a strand of 11-12.
The colors are very even, an unnatural shade of brown, like milk chocolate. The process was started in Japan, but now another company in Geneva, Shanghai Gems SA, is also producing them. Jose Casares did an interview with JNA and told them about the process they use.
Tahitian pearls contain macromolecules of proteinic pigments. They are secreted by the oyster during nacre production. Of the pigments melanin is most important. It is a biopolymer of many forms but only 2 stable - eumelanin, a black polymer, and phaeomelanin, a brown polymer. These determine the color of the Tahitian pearl.
According to Jose Casares of Shanghai Gems (JNA):
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Colour is caused by the absorption and emission of light radiation by electronic double bonds. In our process, we suppress some electronic double bonds in melanin molecules, and the modified melanin appears brown. In other words, we don't add anything but only subtract electrons in specific sites of our pearls.
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The pearls are also heated during the process, and about 20-30% of them will crack. Because of this they usually only use rounds because the nucleus will be in the dead center.
Edit June 11th, 2008
We just decided to add
chocolate pearls.