A saltwater pearl with an unusual appearance was discovered in the pearl market in the city of Zhuji, in China’s Zhejiang Province. It was shaped like a gourd, consisting of a silver gray part and a black part with green overtone (figure 1, left). The pearl’s unusual color and shape raised suspicions about whether it had been artificially processed.
The pearl weighed 2.04 g and had a height of 15.90 mm. The gray part measured 6.79 × 6.12 × 5.07 mm and the black part 10.64 × 10.75 × 10.83 mm. Upon magnification, it appeared the two parts grew together naturally, rather than being artificially attached (figure 1, right).
No silver was detected with energy-dispersive X-ray fluorescence in either part, indicating that the pearl had not been treated with silver nitrate dye. The ultraviolet/visible/near-infrared (UV-Vis-NIR) reflectance spectra of both parts showed absorptions at 279, 405, 500, and 700 nm (figure 2, left), consistent with previous studies on the absorption of saltwater cultured pearls (S. Karampelas et al., “UV-Vis-NIR reflectance spectroscopy of natural-color saltwater cultured pearls from Pinctada Margaritifera,” Spring 2011 G&G, pp. 31–35). In the visible light region, only the cause of the 405 nm band has been identified; it is attributed to a kind of porphyrin called uroporphyrin (Y. Iwahashi et al., “Porphyrin pigment in black-lip pearls and its application to pearl identification,” Fisheries Science, Vol. 60, No. 1, 1994, pp. 69–71). The black part was more absorbed than the gray part due to its richer color.
The Raman spectra displayed bands at 1085 and 704 cm−1 (figure 2, right), corresponding to the ν1 symmetric and the ν4 in-plane bending modes of the carbonate ion (CO32–) in aragonite (J. Unvros et al., “Characterization of some biogenic carbonates with Raman spectroscopy,” American Mineralogist, Vol. 76, 1991, pp. 641–646). Compared to the gray part, several broad bands were observed in the black part at 1800–1400 cm−1, with centers at approximately 1754, 1627, 1527, and 1402 cm−1. These Raman features were attributed to various types of organic components, such as conchiolin and porphyrin, and their intensity clearly increased with the color tone (W. Wang et al., “Identification of ‘chocolate pearls’ treated by Ballerina Pearl Co.,” Winter 2006 G&G, pp. 222–235). These could exclude the possibility of irradiation treatment and other staining treatments.
Under long-wave ultraviolet radiation, both parts exhibited purplish blue fluorescence. However, the gray part exhibited significantly more intense fluorescence than the black part. The 3D fluorescence spectra confirmed this. The strongest fluorescence in the black part was at 336 nm (figure 3, left), while the strongest in the gray part was at 340 nm (figure 3, right). The fluorescence intensity between the two differed by more than 2.5 times. The gray part still had weak fluorescence centers at 432 nm, while the black part did not.
Although the study showed that the pearl’s fluorescence may be attributable to the organic compounds contained within the nacreous layers (J. Hiramatsu et al., “Non-destructive assessment of the effects of heat and sunlight on akoya pearl quality,” Seibutsu Kogaku, Vol. 88, No. 8, 2010, pp. 378–383), the cause of the different fluorescence characteristics is unknown.
The X-ray computed microtomography (μ-CT) scan in figure 4 shows that the outer layer of the two parts was connected by nacre, while the inner layer was composed of voids and organic components. Both parts had a distinct bead nucleus. The organic matter between the bead nucleus and the nacre had a dark gray circular shape. The nacre thickness in both parts was 3–4 mm.
Many mechanisms combine to affect the color of pearls during their growth process (Z. Wang et al., “How cultured pearls acquire their colour,” Aquaculture Research, Vol. 51, 2020, pp. 3925–3934), making it difficult to pinpoint the exact mechanism. This research can be used to further study the relationship between organic compounds and the color and fluorescence reactions of saltwater pearls, excluding variables such as mollusk type and environmental factors.
A saltwater pearl with an unusual appearance was discovered in the pearl market in the city of Zhuji, in China’s Zhejiang Province. It was shaped like a gourd, consisting of a silver gray part and a black part with green overtone (figure 1, left). The pearl’s unusual color and shape raised suspicions about whether it had been artificially processed.
The pearl weighed 2.04 g and had a height of 15.90 mm. The gray part measured 6.79 × 6.12 × 5.07 mm and the black part 10.64 × 10.75 × 10.83 mm. Upon magnification, it appeared the two parts grew together naturally, rather than being artificially attached (figure 1, right).
No silver was detected with energy-dispersive X-ray fluorescence in either part, indicating that the pearl had not been treated with silver nitrate dye. The ultraviolet/visible/near-infrared (UV-Vis-NIR) reflectance spectra of both parts showed absorptions at 279, 405, 500, and 700 nm (figure 2, left), consistent with previous studies on the absorption of saltwater cultured pearls (S. Karampelas et al., “UV-Vis-NIR reflectance spectroscopy of natural-color saltwater cultured pearls from Pinctada Margaritifera,” Spring 2011 G&G, pp. 31–35). In the visible light region, only the cause of the 405 nm band has been identified; it is attributed to a kind of porphyrin called uroporphyrin (Y. Iwahashi et al., “Porphyrin pigment in black-lip pearls and its application to pearl identification,” Fisheries Science, Vol. 60, No. 1, 1994, pp. 69–71). The black part was more absorbed than the gray part due to its richer color.
The Raman spectra displayed bands at 1085 and 704 cm−1 (figure 2, right), corresponding to the ν1 symmetric and the ν4 in-plane bending modes of the carbonate ion (CO32–) in aragonite (J. Unvros et al., “Characterization of some biogenic carbonates with Raman spectroscopy,” American Mineralogist, Vol. 76, 1991, pp. 641–646). Compared to the gray part, several broad bands were observed in the black part at 1800–1400 cm−1, with centers at approximately 1754, 1627, 1527, and 1402 cm−1. These Raman features were attributed to various types of organic components, such as conchiolin and porphyrin, and their intensity clearly increased with the color tone (W. Wang et al., “Identification of ‘chocolate pearls’ treated by Ballerina Pearl Co.,” Winter 2006 G&G, pp. 222–235). These could exclude the possibility of irradiation treatment and other staining treatments.
Under long-wave ultraviolet radiation, both parts exhibited purplish blue fluorescence. However, the gray part exhibited significantly more intense fluorescence than the black part. The 3D fluorescence spectra confirmed this. The strongest fluorescence in the black part was at 336 nm (figure 3, left), while the strongest in the gray part was at 340 nm (figure 3, right). The fluorescence intensity between the two differed by more than 2.5 times. The gray part still had weak fluorescence centers at 432 nm, while the black part did not.
Although the study showed that the pearl’s fluorescence may be attributable to the organic compounds contained within the nacreous layers (J. Hiramatsu et al., “Non-destructive assessment of the effects of heat and sunlight on akoya pearl quality,” Seibutsu Kogaku, Vol. 88, No. 8, 2010, pp. 378–383), the cause of the different fluorescence characteristics is unknown.
The X-ray computed microtomography (μ-CT) scan in figure 4 shows that the outer layer of the two parts was connected by nacre, while the inner layer was composed of voids and organic components. Both parts had a distinct bead nucleus. The organic matter between the bead nucleus and the nacre had a dark gray circular shape. The nacre thickness in both parts was 3–4 mm.
Many mechanisms combine to affect the color of pearls during their growth process (Z. Wang et al., “How cultured pearls acquire their colour,” Aquaculture Research, Vol. 51, 2020, pp. 3925–3934), making it difficult to pinpoint the exact mechanism. This research can be used to further study the relationship between organic compounds and the color and fluorescence reactions of saltwater pearls, excluding variables such as mollusk type and environmental factors.
