A type of pink Japanese akoya cultured pearl, introduced about a decade ago and known as “Sango” pearl (figure 1), uses a pink coral nucleus and a Pinctada fucata mollusk. It is produced by Matsumoto Pearls, a Japanese pearl farming company based in Uwajima, Ehime Prefecture. Sango is the Japanese word for coral, and the species of pink coral used as the nucleus is likely a Corallium species, one of the precious corals harvested off the Pacific coast of Japan, especially around southern Kochi Prefecture (N. Iwasaki et al., “Biology of Japanese Corallium and Paracorallium,” Proceedings of the First International Workshop on Corallium Science, Management, and Trade, 2009, pp. 68–70). Matsumoto Pearls has successfully combined two beautiful organic gem materials to produce these attractive pink-colored pearls.
The coral nuclei in this study exhibited a pinkish orange color and measured 5.60–5.90 mm in diameter. Under microscopic examination, white layer-like structures and banding as well as polyp-related cavities have sometimes been observed. The nuclei also exhibit a weak whitish and stronger whitish fluorescence under short-wave and long-wave UV, respectively. The coral bead’s Raman spectrum (figure 2, top) shows strong pigment peaks at 1129 and 1517 cm–1 and calcite peaks (e.g., 280 and 713 cm–1), identifying them as Corallium species, as in S. Karampelas et al. (“Identification of the endangered pink-to-red Stylaster corals by Raman spectroscopy,” Spring 2009 G&G, pp. 48–52). The full-range photoluminescence spectra closely match that of natural pink coral with natural pigment-related peaks, similar to those in C.P. Smith et al. (“Pink-to-red coral: A guide to determining origin of color,” Spring 2007 G&G, pp. 4–15). Visible-range reflectance spectra (figure 2, middle) reveal peaks at 468, 482, 590, and 666 nm, like those observed in the spectra of natural coral by C.P. Smith et al. (2007). Ten partially drilled Sango pearls examined for this note were of a similar size (5.60–6.00 mm) as the coral beads used as nuclei. No color concentrations were noted down the drill holes or anywhere on their nacreous surfaces using a loupe or microscope. The nacre thickness measured between 120 and 400 microns (0.12–0.40 mm) under the microscope using a small table gauge. The different nacre thicknesses were also visible in real-time X-ray (RTX) images (figure 3). The pearls exhibited excellent luster, with almost no overtone and very weak orient, and showed a weak yellow to greenish yellow fluorescence under long-wave and short-wave UV radiation, respectively. These observations indicated that the 10 samples’ pink coloration was natural, though it remained to be seen whether the pink color originated from the pearls’ nacre or the underlying coral beads.
Chemical analysis using an energy-dispersive X-ray fluorescence (EDXRF) spectrometer revealed manganese levels of 0 to 28 ppmw and strontium levels of 1077 to 1719 ppmw, indicating that the pearls formed in a saltwater environment. A useful observation was that pearl color seemed to be related to nacre thicknesses: Thin nacre overgrowth produced a more obvious pink tint, while thicker nacre resulted in a less saturated pink. Likewise, the Raman and visible-range reflectance spectra were also related to the nacre thickness. Raman spectra for pearls with thin nacre showed stronger pigment peaks at approximately 1129 and 1517 cm–1, similar in strength to those noted in the coral beads; only the aragonite-related peaks—and no pigment peaks—were noted in pearls with thicker nacre (see figure 2, top). The visible-range spectra of the pearls possessing thin nacre also matched the coral beads more closely, albeit with a lower reflectance (figure 2, middle), while pearls with thicker nacre had higher reflectance, as would be expected for lighter-colored pearls. By correlating the coral pigments with the maximum visible reflectance spectrum at 482 nm, the point of least reflectance (see figure 2, bottom), the relationship between nacre thickness and the visible spectra of the pearls could be gauged. The intensity at 482 nm indicates that the pink color of Sango pearls most likely originates from their coral nuclei.
While Sango pearls are not the first cultured pearls to use atypical bead nuclei in the form of other gem materials (see K. Scarratt et al., “Atypical ‘beading’ in the production of cultured pearls from Australian Pinctada maxima,” GIA Research News, 2017, https://www.gia.edu/gia-news-research/atypical-beading-production-cultured-pearls-australianpinctada-maxima), this is the first time the author has analyzed the detailed color origin of Sango pearls with Pinctada fucata mollusks as the host. The pearls are known to possess relatively thin nacre overgrowth, which has enabled the creation of the commercial type of akoya cultured pearl using a natural coral nucleus. GIA would classify Sango pearls as atypical bead-cultured pearls.