The bleaching of P. margaritifera cultured pearls to achieve a brown color has become a popular and widely disclosed treatment (see W. Wang et al., "Identification of ‘chocolate pearls’ treated by Ballerina Pearl Co.," Winter 2006 Gems & Gemology, pp. 222–235). Over the last year, however, the New York laboratory received for identification two strands of cultured pearls (which proved to be from the P. margaritifera mollusk) that featured colors other than brown that showed evidence of bleaching.

Figure 1. The cultured pearls (10.50–13.50 mm) in this variously colored strand are from the Pinctada margaritifera mollusk. All except the greenish ones showed evidence of bleaching.

The cultured pearls in the first strand (figure 1) had good luster, moderately spotted surfaces, and good matching. Those in the second strand (figure 2) had very good luster, lightly spotted surfaces, and very good matching. The color variation in the second strand was broader and more pronounced. In addition to "chocolate pearls," both strands contained cultured pearls in an assortment of atypical colors. Many of the grays and greens were uncharacteristically muted in tone and saturation, and there were a number of unusual light-toned yellow-browns or brown-yellows. Overall, these cultured pearls were lighter in tone and warmer in hue than most P. margaritifera pearls, but darker and cooler than most of those from the Pinctada maxima mollusk. In most of the cultured pearls, magnification revealed evidence of bleaching, including slightly desiccated patches and color distribution (color concentrations and streaks) typical of Tahitian cultured pearls that have been bleached brown, with similar though subtler color variegation in some colors.

Figure 2. All but two of the cultured pearls (11.00–13.50 mm) in this second strand from P. margaritifera also showed evidence of bleaching.

Exposure of the second strand to long-wave UV radiation also produced some unusual results. The tone and intensity of the reaction was directly related to the tone and saturation of the color in the cultured pearls: Lighter samples fluoresced weak-to-moderate yellow, and dark neutrals produced weak dark or inert reactions. Additionally, the reactions varied in hue throughout the strand, and they differed significantly from the inert to weak reddish brown reaction normally produced by naturally colored P. margaritifera pearls. A number of samples showed yellow, orange, or reddish orange reactions, and one fluoresced a distinct green-yellow.

EDXRF analysis of all the cultured pearls in the second strand detected calcium and strontium but no silver, which indicated that they were not dyed. UV-Vis reflectance spectroscopy revealed evidence of bleaching in the browns and all but two of the other colors. The background tilted toward the high-energy side, and the organic peaks were weak and broad (see, e.g., figure 9 of Wang et al., 2006, p. 228). While the level of treatment applied to the other colors visually appeared less pronounced than what is often found in "chocolate pearls," UV-Vis reflectance spectroscopy did not produce any results to corroborate this.

The popularity of "chocolate pearls" seems to have led manufacturers to extend a similar bleaching treatment to cultured pearls of other colors.

Akira Hyatt