Spectroscopic Identification and Characterization of Gemstone Minerals

Since last few years, many artificial and modified gemstones are available whose origins are questionable and even an experienced gemmology expert may be unable to differentiate between these artificial gemstones. Raman spectroscopy is considered to be an effective method to differentiate natural and artificial gemstones as it involves a very simple sample preparation, is non-destructive in nature, and is sensitive, precise, and accurate. In the present study, Raman spectroscopy was employed to identify different gemstones available in the mineralogical collection at the Department of Earth Sciences, Uppsala University. Both precious and semiprecious gemstones like quartz, diamond, beryl, corundum, topaz and tourmaline were evaluated with the usage of a portable Raman spectrometer. The major objective of the present study was to characterize these gemstones and to identify their origin, whether these are natural or synthetic materials, by analysing and comparing Raman spectra. Shifts in dominant peak positions and widths(FWHM) for natural and synthetic samples were extracted in the course of fitting procedures and then compared with the RRUFF database. The variations of these parameters, studied both within the same variety of a gemstone, as well as in between different gemstone varieties, were found to be revealing in several ways. For example, it provided information about the internal residual stresses in quartz crystals, and about the instrumental effects on the spectra. Quartz samples of different colors, shapes, and origins (natural/synthetic) were analyzed. It was found that, within the resolution of this study, it would be difficult to differentiate between synthetic and natural non-metamorphic quartz. A higher resolution study, combined with a chemical analysis by electron microprobe, could possibly facilitate such differentiation. On the other hand, a highly-stressed metamorphic environment leaves an imprint in the crystal structure of quartz, which can be recognized by the Raman technique. For beryl varieties of gemstones, the results suggest that the synthetic crystals display slightly lower values of Raman shifts and smaller values of FWHMs as compared to their natura lcounterparts. This could possibly be used as a criterion for distinguishing between these two categories of beryl gemstones. The agreement between Raman spectra of green tourmalines and those of elbaite from the database was very good, justifying the elbaite identification. The strong correlation of peak positions and FWHMs of studied synthetic diamonds reveals that pressure-temperature and chemistry conditions of synthesis leave a distinct imprint on the crystal structure of diamond, thus making Raman spectroscopy a useful technique for investigating the origin of diamonds, for example detecting their laboratory-specific origin