Identification of natural turquoise imitation with Cu-bearing faustite using raman spectroscopy
| I. Naumenko National Natural Science Museum of the NAS of Ukraine |
| Yu. Gayevsky State Gemmological Centre of Ukraine |
| N. Vovk Internet shop: nvovkjewerly.com.ua |
| Language: Ukrainian |
| Precious and Decorative Stones. No. 1–2 (119–120), 2025: 9–16 |
| Abstract Research on faceted dioptase: optical properties, spectroscopy, chemical composition of mineral inclusions In this article, the authors study two faceted dioptases for the first time in Ukraine, using a classic set of gemological instruments – a refractometer, a polariscope, a spectroscope, a dichroscope and a UV lamp with standard wavelengths of 254 and 365 nm. The results of gemological studies allow us to state that it is quite easy to distinguish dioptase from similar gemstones by a combination of certain refractive indices, very high birefringence, uniaxiality and a positive optical sign, as well as a characteristic absorption spectrum and the absence of luminescence. According to optical parameters, there is only one mineral that is very similar to dioptase – veszelyite, but it is quite rare and is not yet found in faceted form. Also, for one of the studied dioptase samples, it was possible to directly photograph the absorption spectrum in the visible optical range for the first time. The color of dioptase is due to two absorption lines at 14500 cm-1 and 30000 cm-1, which cause both the recognizable bluish-green color of dioptase and the corresponding color of its streak. Studies conducted using IR-Fourier spectroscopy in the IR range of 7000–400 cm-1 allowed us to distinguish certain peaks associated with both the vibration of Si-O-Si bonds (521, 581, 609, 779, 885, 937 сm-1) and the vibrations of water molecules (series 3200–4000 cm-1) in the dioptase structure. It was found that the low resistance of dioptase to heating is associated with the important influence of hydrogen bonding on the formation of the crystal structure of dioptase. In addition, electron microscopic study of the chemical composition of solid inclusions in the studied dioptases allowed us to determine that they are crystals of zinc-containing dolomite. This, together with a comparative analysis of mineral associations of dioptase in different deposits of the world, made taking into account the frequency of findings of the corresponding samples, in turn allows us to make a reasonable assumption about the origin of these dioptases from the Tsumeb deposit in Namibia. |
| Keywords: dioptase, gemological research, refractive index, birefringence, absorption spectrum, IR spectroscopy, dolomite, Tsumeb. |
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