DNA fingerprinting of pearls, corals and ivory: a brief review of applications in gemmology

This article reviews the extraction of DNA (deoxyribonucleic acid) from biogenic gem materials (pearls, corals and ivory) for determining species identification and geographic/genetic origin. We describe recent developments in the methodology adapted for gem samples that is minimally destructive, as well as the successful DNA fingerprinting of cultured pearls from various Pinctada molluscs to identify their species. The DNA analysis methods presented here can also potentially be used for fingerprinting corals and ivory. Biogenic gems—often called ‘organic gems’ (see Galopim de Carvalho, 2018, for a recent discussion of terminology)—are some of the oldest-used gem materials and have been cherished since pre-history (Hayward, 1990; Tsounis et al., 2010; Charpentier et al., 2012). Rather than having a geological origin, these gem materials—such as pearls, precious corals and ivory (e.g. Figure 1)—are products of biomineralisation processes in which living animals produce mineral substances (e.g. calcium carbonate or calcium phosphate) in terrestrial and marine environments (Mann, 2001). Due to their importance in jewellery and decorative arts, the study of biogenic gem materials constitutes an important part of gemmological research. Natural pearls form in wild molluscs without any assistance, whereas cultured pearls are the result of human intervention on cultivated pearl-producing molluscs (Strack, 2006; Hänni, 2012). Pearls and their shells consist of secretions of different polymorphs of calcium carbonate (CaCO3) such as aragonite, calcite and vaterite. Pearls are sometimes coloured by organic pigments. Precious corals have not been cultivated commercially so far, and those used in jewellery and objets d’art represent the coral skeleton (secreted by living polyps), which consists of CaCO3 as well as protein, glycosaminoglycans and proteoglycans (Debreuil et al., 2012). They can be coloured by carotenoids and other types of pigments. Finally, elephant ivory from African (Loxodonta spp.) and Asian (Elephas spp.) elephant tusks is comprised of collagen and carbonate-rich hydroxyapatite (dahllite, Ca10[PO4]6[CO3] • H2O; Edwards et al., 2006). Ivory can be found in a large number of animal species, of which elephant ivory is the most studied due to its value, recognition and cultural importance. In recent years, fossilised mammoth ivory has appeared more widely on the market, as elephant ivory trade restrictions have taken force (e.g. under the Convention on International Trade in Endangered Species of Wild Fauna and Flora, or CITES; www.cites.org/eng/niaps). CITES regulates the trade in biogenic gem materials that are produced by species included in its Appendices I, II or III. Among these are various species of precious coral, queen conch (pearls) and giant clam (pearls). The ability to trace biogenic gem materials back to their species-related and geographic origins can provide greater transparency and help curb trade in illegal materials (and thus restrict poaching and smuggling). Furthermore, such research can yield important information The Journal of Gemmology, 36(2), 2018, pp. 152–160 http://dx.doi. org/10.15506/JoG.2018. 36.2. 152 © 2018 The Gemmological Association of Great Britain