Identification of Irradiated South Sea Cultured Pearls Using Electron Spin Resonance Spectroscopy

GEMS & GEMOLOGY WINTER 2012 I dyeing, bleaching, and heat treatment are widely used methods to alter pearl color. Although most artificial colors are easily recognized, some resemble attractive colors that occur in nature (Elen, 2001; Li and Chen, 2001; Zachovay, 2005; Wang et al., 2006; “Better techniques improve brown pearls,” 2006; McClure et al., 2010). Lower-quality freshwater and saltwater cultured pearls are regularly exposed to 60Co gamma-ray radiation in an attempt to simulate black pearls or enhance orient (Crowningshield, 1988; Li and Chen, 2002; O’Donoghue, 2006). In recent years, the irradiation process has been applied to not only Akoya cultured pearls and freshwater cultured pearls (FWCPs), but also to South Sea cultured pearls (SSCPs) (Choi et al., 2012). The irradiation-induced color change results from the darkening of the nucleus, caused by MnCO3 oxidation, as well as denatured damage to the pearl’s conchiolin (Matsuda and Miyoshi, 1988). FWCPs have a higher abundance of proteinous components and manganese than saltwater pearls (Hatano and Ganno, 1962). Gamma-ray irradiated SSCPs (figure 1) were first discovered in the Korean market in April 2011. At the March 2011 Hong Kong Jewelry Show, a Japanese trader reportedly sold a Korean counterpart irradiated SSCPs without disclosing the treatment. They were light gray or silver loose cultured pearls and beads 10–16 mm in size. While a cream, yellow, or black color is produced by a protein pigment in the nacre, a blue or silver color is caused by organic material between the nacre and nucleus (Komatsu, 1999; O’IDENTIFICATION OF IRRADIATED SOUTH SEA CULTURED PEARLS USING ELECTRON SPIN RESONANCE SPECTROSCOPY