Sango pearl and the color origin

Abstract

Sango pearl is the trade name for pink cultured Japanese Akoya pearls developed using pink coral beads by one Japanese pearl farming company in Uwajima, Ehime prefecture (Matsumoto Pearl). The pink coral used is considered as the precious coral Corallium elatius that is harvested along the Pacific coast of Japan, especially the coast of the Kochi prefecture (1). Sizes of ten partially drilled Sango pearls examined were 5.60–6.00 mm in diameters. No color concentrations were observed down the drill holes or anywhere on their nacreous surfaces using a microscope. The nacre thickness measured between 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 images. Expectedly the pearl colors seemed to be related to nacre thicknesses: Thin nacre produced a stronger pink hue, while thicker nacre resulted in a lighter pink. The UV-Vis reflectance spectra were also related to the nacre thickness. The UV-Vis spectra of the pearls possessing thin nacre also matched the coral beads more closely, albeit with a lower reflectance, while pearls with thicker nacre revealed 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, 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.