Iridescent Gems Cut from Hinge Ligament of South Sea Pearl Oyster (Pinctada maxima)
Pearl oysters of the Pinctada genus are known to produce beautiful nacreous pearls (The Pearl Blue Book, CIBJO, 2022). The iridescent nacre of their shells, known as mother-of-pearl, is often used as a gem material. Another part of the oyster—the hinge ligament—also can be used as a gem material. The hinge ligament is a mineralized tissue that connects the two shells of a bivalve mollusk.
Shoji Naito, a Tokyo-based lapidary with more than forty years of experience in gemstone cutting, had a large South Sea pearl oyster (Pinctada maxima) shell that he kept for at least 37 years. The cross section of the shell’s thick hinge ligament displayed iridescence. To enhance this attractive phenomenon, the hinge ligament was cut and polished into cabochons, a practice that has remained unknown in the gem industry. Author YK recently purchased one of these cabochons: a 0.72 ct black opaque specimen.
Due to the item’s appearance, particularly its greasy luster and iridescence, the cabochon can be confused with other iridescent materials such as opal imitations. Since this unique material has yet to be studied in detail, Naito loaned rough and polished cabochon ligaments as well as the shell (figure 1) to GIA for gemological examination.
The 0.72 ct cabochon had a refractive index of 1.560 (spot reading) and a hydrostatic specific gravity (SG) of 1.18. The very low SG of this cabochon compared to pearls or shells, which are generally above 2.60, indicated that it contained a higher percentage of organic material. Microscopic observation revealed fibrous structures with iridescent reflections. The fibrous structures displayed a wavy pattern of blue/green-dominant iridescence against the black opaque matrix. Rough fragments showed the same patterns at the cross sections (figure 2).
In addition to standard gemological testing and observations, more advanced testing methods were applied to the fragment and polished samples at GIA’s laboratory in Carlsbad. Raman spectroscopy with 830 nm laser excitation clearly detected aragonite peaks (1086, a doublet at 702 and 706, 206, and 152 cm–1). The ultraviolet/visible/near-infrared (UV-Vis-NIR) reflectance spectra showed an absorption at 280 nm associated with conchiolin. For collecting Fourier-transform infrared (FTIR) spectra, two KBr pellets were prepared with powders collected from ligament fragments. The FTIR spectra (figure 3) showed aragonite peaks (1486, 1082, 855, 713, and 700 cm–1) and an amide I peak at 1646 cm–1 related to conchiolin, which are identical to the spectrum reported for the ligament of the akoya pearl oyster (P. fucata) (M. Suzuki et al., “A unique methionine-rich protein-aragonite crystal complex: Structure and mechanical functions of Pinctada fucata bivalve hinge ligament,” Acta Biomaterialia, Vol. 100, 2019, pp. 1–9).
Energy-dispersive X-ray fluorescence (EDXRF) spectroscopy showed very high calcium (397300 ± 1600 ppm), low levels of manganese (110 ± 42 ppm), and high strontium (3077 ± 89 ppm) contents for the 0.72 ct cabochon, confirming calcium carbonate material of saltwater origin. The results for the fragments also showed the same trend. Laser ablation–inductively coupled plasma–mass spectroscopy (LA-ICP-MS) analysis confirmed the EDXRF results. Furthermore, most trace elements exhibited concentrations comparable to previously reported nacreous P. maxima pearls (e.g., K. Scarratt et al., “Natural pearls from Australian Pinctada maxima,” Winter 2012 G&G, pp. 236–261; N. Sturman et al., “Bead-cultured and non-bead-cultured pearls from Lombok, Indonesia,” Fall 2016 G&G, pp. 288–297). The magnesium level was significantly higher, however, which is likely related to the organic-rich nature of the material.
Real-time X-ray microradiography (RTX) revealed that both the cabochons and fragments exhibited a similar banded structure alternating between light gray and dark gray stripes. The light gray areas appeared to be dense aragonite material that was generally more radiopaque than the dark gray areas of protein-rich materials (figure 4). Cracks were presented as darker gray lines across the structure. The internal banded structure corresponds to the arrangement of the fibers creating the iridescent effect.
The hinge ligaments of Tahitian pearl oyster (P. margaritifera) shells from GIA’s research collection were also tested for comparison. Due to the smaller size of the P. margaritifera shell, the hinge ligaments are thinner and display less iridescence than the P. maxima sample studied. However, the testing results were consistent.
This study showed that FTIR (KBr pellet) analysis can be used, in addition to standard gemological testing and microscopic observations, to verify the characteristic aragonite and amide I peaks of the hinge ligament. However, KBr pellet analysis is destructive—Raman and UV-Vis-NIR reflectance spectroscopies and RTX analysis should be attempted first.
This unique biogenic material from pearl oysters can potentially produce a new eye-catching blue-green iridescent gem material.
