Identification of “Chocolate Pearls”

[주임교수유동일]

Figure 1.  The cultured pearls in this
12.0-13.7 mm “chocolate pearl” necklace
and 12.9 mm ring exhibit intense brown
colors. In natural-color Tahitian cultured
pearls, dominant brown coloration is rare.
Courtesy of Emiko Pearls International;
photo by Robert Weldon, ©GIA.

Identification of “Chocolate Pearls”
Treated by Ballerina Pearl Co.
© Gemological Institute of America. May 2007. All Rights Reserved.
By Wuyi Wang, Kenneth Scarratt, Akira Hyatt, Andy Hsi-Tien Shen, Matthew Hall, and Robert Weldon.

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. The present article is a summary of a comprehensive report on this material that appeared in the Winter 2006 issue of GIA’s professional journal, Gems & Gemology.

Figure 2.  When the 10.0 mm “chocolate” cultured pearl on the left was cut to a depth of 1 mm, the induced brown coloration appeared homogenous through the nacre. A much deeper cut toward the nucleus of the 10.8 mm yellow-brown CCP on the right also reveals a homogenous coloration throughout the nacre, though a lighter tone can be seen at the interface with the bead nucleus. 
Photo by Jian Xin (Jae) Liao, ©GIA.

The Study

Ballerina Pearl Co. and its distributor, Emiko Pearl International, supplied 160 “chocolate” cultured pearls (CCPs) and three silver-dyed Tahitian cultured pearls (DTCPs) for this study. We also examined 29 known natural-color Tahitian cultured pearls (NCTCPs) from the GIA Collection, and had an additional four NCTCPs treated specifically for this study and examined before, during (“preparatory treated,” three samples), and after processing. In total, we looked at 196 cultured pearls (ranging between 3.75 ct and 21.40 ct).

We used standard gemological observation techniques (including microscopy and reaction to UV radiation), as well as other tests requiring more sophisticated instrumentation, to study these cultured pearls. In addition, we cut through the nacre of two “chocolate pearls” to determine the degree to which the color varied with depth.

Visual Observations − Most of the natural-color cultured pearls exhibited a greenish gray to dark greenish gray coloration, which was not always homogenous. Most also had overtones of violet, blue, yellow, green, and rosé, and medium-to-high luster.

The “chocolate pearls,” conversely, were dominated by a brown coloration, though some secondary colors showed up as well. Little or no orient was noted, but the luster was also medium to high. The sample CCPs from which slices had been removed showed very even nacre coloration; however, a lighter color was noted near the nucleus interface when one pearl was deeply cut. Finally, in the silver-dyed category, dark brown colors and pink overtones with medium luster were noted.

One of the principal areas of difference between the various sample types was their reaction to long- and short -wave ultraviolet radiation, as follows:

Long-wave (strongest differences noted):

• The gray and brown natural-color Tahitian cultured pearls fluoresced a very weak orange, yellow, or greenish yellow.
• Almost all the “chocolate pearls ”fluoresced" a weak to moderate chalky reddish orange, with an appearance that was not seen in their natural-color counterparts.
• The silver-dyed samples showed no reaction to either long- or short-wave UV radiation.

Short-wave:

• The “chocolate pearls” generally fluoresced a weak greenish yellow – a feature that was seen in many of the untreated samples as well, though even weaker in intensity.

Infrared Spectroscopy – No major differences were seen, at least none that alone could help separate the different pearl categories. All readings showed characteristic absorption peaks for aragonite, a principal component of pearl nacre.

UV-Vis-NIR Reflectance Spectroscopy – The absorption bands recorded from the “chocolate pearls” were similar in placement to those of their natural-color counterparts, but they were distinctly weaker and much less defined; they were also shifted to higher wavelengths in the Caps. Substantial differences in spectral baseline were observed as well.  The absorption spectra of the dyed samples had few similarities to those of their natural counterparts.

Raman and Photoluminescence Spectroscopy – All of the pearls exhibited strong photoluminescence. When normalized, however, the photoluminescence was stronger in the “chocolate pearls” than in the NTSC’s. It was weakest in the silver-dyed cultured pearls.

Chemical Analysis – The main difference noted was the silver content in the dyed cultured pearls, which was not seen in the CCP and NCTCP readings. Although the “chocolate” and natural-color cultured pearls appeared very similar chemically, the Caps did exhibit a significantly lower concentration of potassium than the NTSC’s.

Figure 3.  Observations (before and after) of four of the Tahitian cultured pearls GIA submitted for treatment by Ballerina Pearl Co.

Conclusions

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.

Figure 4.  Several companies are producing cultured pearls with “chocolate” colors, so it can be assumed that more than one process is being used to achieve those colors. This study focused on the proprietary two-step process used by Ballerina Pearl Co. These 11.0-15.7 mm cultured pearls are courtesy of Emiko Pearls International; photo by Robert Weldon, ©GIA.

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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