CAUSE OF COLOR ON POLYNESIAN BLACK PEARLS
As discussed above, color in pearls is a mixture of body color and overtones. The body color is determined by a combination of factors, including the biology of the molluslz (specifically the mantle tissue), the composition of the mother-of-pearl shell, and trace elements present in the water environment. Japanese researchers have investigated the body color in pearls extensively for over 50 years (Fox, 1979). They cite the presence of porphyrins (a group of water-soluble, nitrogenous 16-member ring organic compounds) in the shell of the molluslz as a primary cause of color in colored oyster pearls.
In inolluslzs, the porphyrins combine with metals such as lead and zinc to form metalloporphyrins. These same porphyrins produce a red fluorescence that is useful in identifying natural color in blaclz cultured pearls. Miyoshi et al. (1987) illustrate the diagnostic spectra produced by porphyrins present in the nacre. Also contributing to the color of most blaclz pearls is the presence of brownish organic substances that exist between the translucent porphyrin- containing nacre and the bead nucleus (Miyoshi et al., 1987; l? Galenon, pers. comm., 1989). This substance is thought to be conchiolin, but research to date has not been conclusive. Fritsch and Rossman (1988) describe the cause of the “high order” interference colors -overtone and orient -seen in black pearls as “light passing through and reflected back by alternating layers of aragonite [in the nacre] and conchiolin.” The finer the layers of nacre are, the more orient a pearl has (R. Wan, pers. comm., 1989).
In a 1971 article, C. Denis George lamented his unsuccessful search for even one natural-color black pearl in visits to Mexico City, New York, and Paris. He was routinely offered treated blaclz pearls that were represented to be natural color, and he railed against the “unscrupulous suppliers” who were perpetrating this “miserable and fraudulent” situation. In fact, from 1900 to 1978 (when cultured black pearls first began to appear in quantity), there were far more treated than natural-color black pearls on the market. One result of the overharvesting of I? margaritifera in the 19th century was that by 1900 there was a shortage of natural-color blaclz pearls. To fill this void, people began to use silver nitrate solutions to dye the smaller Alzoya pearls common to Japan (figure 19).
Even today, silver nitrate and other silver salts are probably the most common form of treatment to turn white and off-color Akoya pearls black (Komatsu and Alzamatsu, 1978; Taburiaux, 1985). Although pearl treaters are among the most secretive in the gem industry, we do know that the basic procedure involves soaking the pearls in a weak solution of silver nitrate and dilute ammonia and then exposing them to light or hydrogen sulfide gas. This produces a change of color in the conchioliii that makes the pearl appear black in reflected light. The resulting color is stable to light and heat (Nassau, 1984). Because the hues of brown-black, green-black, and blaclz are similar to natural colors, it is virtually impossible to distinguish them by visual observation alone (Taburiaux, 1985).
Another, reportedly organic, dyeing technique was commonly practiced from approximately 1915 through the 1920s. Called the French Method, it was used by a few treaters in Paris on off-color natural pearls. Although little is recorded about the actual procedure, we do know that it can be detected with a microscope when dye concentrations are present. Pearls were shipped from Japan to Paris for treatment and then back to Japan for sale (R. Crowningshield, pers. comm., 1989). In 1920, Rosenthal cautioned jewelers to be aware of pearls treated by this process. Although historically treatment has involved Akoya cultured pearls, it was inevitable that attempts would be made to enhance light-color P. maxima and P. margaritifera cultured pearls as well. In 1987, Fryer et al. reported seeing a strand of 11- to 14-mm blaclz cultured pearls that showed evidence of silver nitrate dye. More recently, in September 1989, the GIA Gem Trade Laboratory in New York examined two 12-mm blaclz cultured pearls that showed evidence that they might have been dyed.
The laboratory staff subsequently received confirmation from the trade that some South Sea pearls were being treated to darken the color (D. Hargett, pers. comm., 1989). One of the more recent treatments used on P, fucata martensii (Alzoyas) in an effort to darken mediocre-color cultured pearls is irradiation, specifically with a cobalt gamma source. According to Matsuda and Miyoshi (1988), gamma-ray irradiation can change off-color cultured Alzoya pearls to an attractive bluish gray. These authors report that irradiation of Alzoya pearls began in the 1950s with the ‘Atoms-for-Peace Program” and resulted in irradiated cultured pearls first appearing on the market in the 1960s. Ken Tang Chow’s patent on irradiating pearls, filed in 1960 and granted in 1963, sheds some light on the procedure used. The technique he patented involves exposure of the pearl to cobalt-60 with an intensity of 1,000 curies of gamma rays at a distance of 1 cm from the source for about 20 minutes at room temperature. Chow found that longer periods of irradiation did not produce any appreciable change in color. He also reported that the irradiated pearls were stable to light and heat.
Scientists have often noted that the color of freshwater shells and pearls can be changed by irradiation more easily than that of saltwater oysters. They attribute this to a change of manganese compounds (MnC03+MnO) which are more abundant in freshwater mollusks (Wada, 1981). Irradiating Alzoyas produces a darkening of color because the freshwater bead nucleus darkens and influences the body color. In P. margaritifera, the much thicker nacre would not allow the color shift of the nucleus to be visible (R. Crowningshield and D. Hargett, pers. comm., 1989).
Dr. George Rossman, of the California Institute of Technology, recently experimented with the irradiation of three Polynesian blaclz pearls following the procedure outlined in Chow’s patent, but left them in the reactor for slightly longer than 24 hours. No appreciable change was observed in these pearls compared to their control samples, although a color shift was observed in the freshwater pearls irradiated at the same time (pers. comm., 1989).
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