odern pearl identification-including the type of pearl and any treatment applied-is performed by applying almost exclusively nondestructive methods. These include macro to microscopic observations, spectroscopy, chemical analysis, fluorescence, and various forms of X-ray imaging (Scarratt, 2011). Surface inspection differentiates nacreous from non-nacreous pearls, while chemical analyses using X-ray fluorescence and luminescence can distinguish between saltwater and freshwater pearls (e.g., Hänni, 2000; Hänni et al., 2005;Kessrapong et al., 2017). Internal inspection using X-radiography, which was introduced to gemology in the 1930s (Dirlam and Weldon, 2013), has evolved into high-tech X-ray computed microtomography (μ-CT). This technology enables more consistent separation of natural, non-bead-cultured, and bead-cultured pearls (e.g., Sturman, 2009;Karampelas et al., 2010).One of the ongoing challenges is to identify the saltwater mollusk species producing white, cream, or silver nacreous pearls, since these can form in all Pinctada species as well as some other species, and can have similar appearances. As pearls are formed by mollusks, a direct identification method would be to find biological evidence such as DNA indicators. (For boldfaced terms, please refer to the glossary on pp. 48-49.) DNA studies emerged in the biological sciences (e.g., Toro, 1998;Masaoka and Kobayashi, 2004; Freier et al., 2008) after the development during the 1980s of the polymerase chain reaction (PCR) method of duplicating gene fragments (see figure 1A). A number of studies on pearl oysters of the Pinctada species (e.g., Masaoka and Kobayashi, 2006;Tëmkin, 2010;Masaoka et al., 2016) have investigated their nuclear and mitochondrial ribosomal ribonucleic acid (rRNA) and the internal
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