Spectroscopic (confocal micro-raman and FT-IR) and microscopic (immersions and gemmological microscopes) investigations of the gemstone polished blue topaz

The mineral topaz [Al2SiO4(F,OH)2] is a precious gemstone. In this study, natural untreated, natural irradiated, and synthetic untreated topaz samples were examined. In addition, both artificial different materials and natural untreated colorless topaz samples were also compared with them. In the topaz, one of the fluorine (F-1) and hydroxyl (OH-1) ions is present in more abundance in the crystal lattice than the other. The abundance of fluorine ions is of particular importance in the blue topaz samples. Because the florine-rich colorless or fancy colored topaz samples can be easily converted to saturated blue when subjected to proton, electron and/or neutron irradiation at certain time and doses. Since this color conversion is more efficient than the hydroxyl-rich ones, the determination of the fluorine richness in the topaz samples by spectroscopic methods is important before the irradiation treatment. Blue topaz samples, which are facet cut and polished, were examined by using dispersive confocal micro-Raman (DCµR) and Fourier Transform Infrared (FT-IR) with non-destructive spectroscopic devices. In the DCµR graphics, the natural and untreated blue topaz samples that from the main bands with the strongest peak intensity, those of the bands at 265, 236, 329 ve 404 cm-1 may be attributed to stretching bands as molecular vibrations belonging to (AlO4), those of the bands at 285, 453 and 481 cm-1 may be attributed to stretching bands as molecular vibrations belonging to (SiO4), those of the bands at 926 and 521 cm-1 may be attributed to bending bands as vibrations belonging to (F), and finally, those of the bands at 852 and 1461 cm-1 may be attributed to bending bands as vibrations belonging to (OH). Since, the intensity of these bending vibration bands can be directly related to the abundance of the fluorine presence. The resulting FT-IR graphics show that the peak intensity and morphology of bands in the ranging of 3100-3800 cm-1 indicate the presence and abundance of water as the hydroxyl group. They have been found to be very low, obviously in fluorine-rich blue topaz samples. Keywords: Gemstone Blue Topaz, Florine (F-1) - Hydroxyl (OH-1) content, Gemology, Forensic Gemology, DCµR and FT-IR Spectroscopy