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Identification of “Chocolate Pearls” treated by Ballerina Pearl Co
The study found no evidence of a foreign coloring agent in the Caps
By: Diamond World News Service
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May 9 2007 12:00AM
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Reference: 1373  

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A new type of cultured pearl began entering world markets in 2000. Because of their attractive brown coloration, they came to be known as “chocolate pearls.” Some in the trade press suggested that the unusual hues were the result of irradiation treatment of Tahitian black cultured pearls, but at least two of the companies known to perform the treatment, Ballerina Pearl Co. in New York and Shanghai Gems SA in Geneva, maintained that no color was being added.

Ballerina Pearl Co. agreed to provide the Gemological Institute of America (GIA) with samples of their “chocolate pearls” and to treat samples for before-and-after comparison, so GIA researchers could both learn more about this process and establish identification criteria.

One of the main methods of identifying “chocolate pearls” is visual: Natural-color brown cultured pearls rarely show the intensity or uniformity of color exhibited by their “chocolate” counterparts. Natural-color brown cultured pearls also show more orient and/or rosé overtone than do Caps.

It is generally understood that pearl colors are produced by organic components between the calcium carbonate (aragonite) platelets of which pearl nacre is made. Our study suggested that these components are altered during treatment. The alteration is indicative of a bleaching process, resulting in the “chocolate” colors. While the mechanism is not fully understood, this alteration may be responsible for the characteristic differences in long-wave UV fluorescence between the natural-color and treated “chocolate” cultured pearls.

The study also found no evidence of a foreign coloring agent in the Caps, which is invariably present in the silver-dyed pearls. Because silver dye strongly absorbs light, dyed pearls are inert to short- and long-wave UV radiation. UV-Vis-NIR reflectance spectroscopy also provides good evidence that a bleaching process occurs with the Caps and, if the instrumentation is available, supplies information that is useful for identification.

Although it is likely that the various producers of “chocolate” cultured pearls worldwide use more than one method to induce a brown color in their products, this study provided important answers for the characterization and identification of Ballerina “chocolate pearls.”

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