Lab Notes Gems & Gemology, Spring 2024, Vol. 60, No. 1

Irradiated and Dyed Akoya Pearls


Figure 1. The two groups of reportedly irradiated akoya pearls used in this study. Photos by Sood Oil (Judy) Chia.
Figure 1. The two groups of reportedly irradiated akoya pearls used in this study. Photos by Sood Oil (Judy) Chia.

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.

Figure 2. Images showing obvious color concentrations at the drill holes of some samples with darkened bead nuclei (A and B), an unnatural dark patchy surface (C), and a darkened bead nucleus without color concentration (D). Photomicrographs by Emiko Yazawa; fields of view 4.79 mm (A and C) and 3.57 mm (B and D).
Figure 2. Images showing obvious color concentrations at the drill holes of some samples with darkened bead nuclei (A and B), an unnatural dark patchy surface (C), and a darkened bead nucleus without color concentration (D). Photomicrographs by Emiko Yazawa; fields of view 4.79 mm (A and C) and 3.57 mm (B and D).

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).

Figure 3. Left to right: Cross sections of an irradiated akoya pearl, a white akoya pearl, and an irradiated and dyed akoya pearl. Photo by Emiko Yazawa; field of view 14.52 mm.
Figure 3. Left to right: Cross sections of an irradiated akoya pearl, a white akoya pearl, and an irradiated and dyed akoya pearl. Photo by Emiko Yazawa; field of view 14.52 mm.

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.

Figure 4. Raman spectra of one sample’s surface nacre, nacre on the cross section, and bead nucleus, along with the surface nacre of a white akoya pearl for comparison. Unusual peaks were found in both nacre areas but not on the bead nucleus. Also shown are typical aragonite features at 702, 704, and 1086 cm<sup>–1</sup>.
Figure 4. Raman spectra of one sample’s surface nacre, nacre on the cross section, and bead nucleus, along with the surface nacre of a white akoya pearl for comparison. Unusual peaks were found in both nacre areas but not on the bead nucleus. Also shown are typical aragonite features at 702, 704, and 1086 cm–1.

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.

Emiko Yazawa is a senior analytics technician, and Chunhui Zhou is senior manager of pearl identification, at GIA in New York.