Treated Freshwater Non-Bead Cultured Pearls with an Antique Appearance
GIA’s Mumbai laboratory recently received for identification a group of nine drilled and three undrilled pearls weighing 44.37 carats total and ranging in size from 7.38 × 6.49 × 6.05 mm to 10.61 × 9.05 × 8.05 mm. The nine drilled pearls appeared to be old, while the three undrilled pearls looked whiter and much newer overall (figure 1). The shape of the samples varied, and the nine drilled pearls possessed a notable “aged” yellow coloration, which was also evident on some areas of the three undrilled whiter pearls.
The older-looking pearls possessed a dull luster, but the nacre condition was good. Microscopic examination at 70× magnification soon revealed that the color was concentrated on the outer layers and within surface-reaching features, proving that the color was not natural and a treatment had been used to alter their appearance (Summer 2017 Gem News International, pp. 255–256).
Although the color of pearls can change over time due to various causes (e.g., care factors such as storage conditions and contact with chemicals), these changes tend to be very gradual and develop over many years. This is partly due to their biogenic composition, which mainly consists of calcium carbonate with traces of organic substances, residual substances, and water. Dry environments are generally not advisable as they may result in surface alterations and, in rare cases, weight loss. The color can also change over time through wear and tear and the accumulation of external contaminants, so further analysis was required to prove whether these pearls naturally discolored over time or were treated to look antique, thereby inflating their value (M.S. Krzemnicki, “Fake historic provenance: ‘Aged’ cultured pearls,” Facette, No. 25, 2019, p. 28).
Visually, it was readily apparent that the drill holes were very large in relation to the size of the pearls. When viewed with a 10× lens or under a gemological microscope at 70×, color concentrations were observed around the drill holes of all nine drilled pearls. The surfaces were also heavily worked around the drill holes and did not resemble the wear and tear that might be expected on antique pearls. The surfaces of the nine drilled samples (2–4 and 7–12) were also slightly etched, indicating possible exposure to a mild acidic solution to create an “aging” effect. Strikingly, the color was not typical of natural-color pearls (Summer 2017 Gem News International, pp. 255–256) and appeared to be restricted to the surface and around the drill holes (figure 2). Areas on some pearls also revealed the original underlying white color, further proof of treatment (figure 3).
Next, we needed to determine whether the pearls originated from a saltwater or freshwater environment, and whether they were natural or cultured. Real-time microradiography revealed small central twisting void-like features surrounded by fine growth lines typical of freshwater non-bead cultured pearls (figure 4), consistent with those produced by Chinese farms (K. Scarratt et al., “Characteristics of nuclei in Chinese freshwater cultured pearls,” Spring 2000 G&G, pp. 98–109). The freshwater origin was confirmed when the pearls fluoresced a strong yellowish green color upon exposure to X-ray fluorescence, and further substantiated by manganese values ranging from 500 to 1700 ppm and strontium values from 800 to 1000 ppm, obtained by energy-dispersive X-ray fluorescence spectroscopy.
Since Raman spectroscopy is also a valuable analytical tool for differentiating natural versus treated color in pearls, 514 nm ion-argon laser excitation was used on the surfaces. The results revealed a doublet at 702 and 705 cm–1, as well as a peak at 1085 cm–1 indicative of aragonite, which is seen in the majority of pearls. High background fluorescence, often characteristic of treated-color pearls, was also noted in the drilled pearls. The photoluminescence spectra obtained displayed high fluorescence more typical of treated pearls, as well as the expected aragonite peaks. Other than the suspiciously high fluorescence, no other peaks characteristic of dyeing were observed.
All observations and results indicated that the nine drilled pearls had been treated to make them look antique. Pearls 1, 5, and 6 were predominantly white and showed only minimal evidence of any artificial aging attempts. These three showed a reaction to long-wave UV fluorescence more indicative of routine processing carried out on freshwater pearls. We concluded that these freshwater non-bead cultured pearls were also treated to make them appear aged. Hence, the report stated that these were freshwater non-bead cultured pearls from the Unionidae family and that pearls 2–4 and 7–12 had been color modified. Since the majority of the surfaces of pearls 1, 5, and 6 remained white and only showed insignificant areas of discoloration, the modified color description was not applied.
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