An Analysis of Brick-colored Pearls Harvested from Japanese Pearl Oysters Infected with Akoya Oyster Disease

摘要

Pearl oysters, infected by Akoya Oyster Disease, have a high likelihood of producing brick-colored pearls, known as Nigori Tama. NMR analysis of Nigori Tama showed the presence of melanin and fatty acid peroxides. Normally these organic matters can bedisintegrated and removed from the pearls by bleaching with hydrogen peroxide solution and methanol under fluorescent lamp irradiation by the professional bleaching companies. However, even slightly brick-colored Nigori Tama was not bleached by this process. Additionally scanning electron microscopic observations revealed that the prismatic layer of Nigori Tama was significantly thicker than that of the normal pearls. Therefore we conclude that the thick prismatic layer is the cause of the brick-colored pearls.