Irradiated and Dyed Akoya Pearls
Since the 1960s, irradiation has been a known treatment for modifying the color of freshwater cultured pearls (Developments and Highlights at the Gem Trade Lab in Los Angeles, Spring 1967 G&G, pp. 153–154). It was found that freshwater shells and pearls contained higher amounts of manganese, which could be oxidized by irradiation to produce a darkened color (T. Tsujii, “The change of pearl colors by the irradiation with γ-ray or neutron ray,” Journal of Radiation Research, Vol. 4, No. 2-4, 1963, pp. 120–125). While saltwater pearls contain much less manganese than freshwater pearls, cultured saltwater akoya pearls could also be treated with irradiation. During treatment, the freshwater shell bead nuclei inside these cultured pearls would turn darker and cause the surface color, luster, and overtone to change (Winter 1988 Gem Trade Lab Notes, p. 244).
Recently, GIA’s New York laboratory obtained two groups of reportedly irradiated akoya pearls from two different vendors. These samples were either drilled or partially drilled, ranging from 3.82 mm to 8.15 mm in diameter and from 0.29 to 3.58 ct in weight (figure 1). They exhibited a variety of light to dark-toned gray with bluish and greenish bodycolors, with variously colored overtones.
Real-time X-ray microradiography and energy-dispersive X-ray fluorescence confirmed that these were bead cultured pearls grown in a saltwater environment. No silver content was detected on any of the surfaces, suggesting that the colors of these pearls had not been modified by silver nitrate treatment. Upon microscopic examination, however, some of the samples displayed obvious color concentrations at their drill holes or unnatural patchy surface color distributions, while also revealing a darkened bead nucleus inside. On the other hand, some light-colored pearls did not possess color concentrations at the drill hole and only contained a darkened bead nucleus (figure 2).
To further investigate the color origin, we cut several samples as well as a typical white akoya cultured pearl to compare their internal appearances (figure 3). The results confirmed that the reportedly irradiated pearls contained darkened bead nuclei showing brownish stripes rather than the usual white bead nucleus found in a white akoya pearl, indicative of irradiation treatment (Winter 1988 Gem Trade Lab Notes, p. 244). In addition, some samples exhibited a distinctly different color appearance at the nacre and drill-hole areas, indicating dye treatment.
Short-wave UV fluorescence spectroscopy confirmed that the akoya pearls may have been treated by any number of processes (C. Zhou et al., “Detection of color treatment and optical brightening in Chinese freshwater ‘Edison’ pearls,” Summer 2021 G&G, pp. 124–134), based on low fluorescence counts at around 320–360 nm. Raman spectroscopic analyses using 514 nm laser excitation revealed aragonite as the main component. Some treated pearls exhibited a series of unusual Raman shift peaks between 560 and 3200 cm–1 on the nacre (probably due to dye materials) but not on the darkened shell bead nucleus, suggesting two forms of treatment: dyeing and irradiation (figure 4).
Naturally colored gray pearls are usually light in color and tend to show uneven color distribution, making them difficult to match. Some grayish akoya pearls on the market have been treated to imitate these natural colors, since they command higher prices. While dye treatments are more easily detected by unnatural bodycolors, obvious color concentrations, and advanced testing, irradiation treatment is harder to detect since the nacre is not affected and the difference in bodycolor is subtle. Careful visual observation is necessary. The combination of irradiation and dye treatments in some of these treated pearls is not commonly encountered in our laboratory.
