Identification Characteristics of Gem-Grade Natural Glass

There are two kinds of gem－grade natural glass in daily inspection，obsidian and tektite，which have similar appearance.It is difficult to distinguish them.Obsidian and tektite are formed under different geological environments，and possess different physical and chemical characteristics.By using the conventional gemmological tests as well as scanning electron microscope（SEM），energy dispersive spectrometer（EDS），and Fourier transform infrared spectrometer（FTIR），this article contrasted the physical properties and chemical compositions of obsidian and tektite.
Both tektite and obsidian are mainly composed of SiO₂, and Al₂O₃. The content of SiO₂.in tektite is about 71.3%~79. 3%, while SiO₂, of obsidian is about 70%. Otherwise, FeO_T in obsidian is less than that in tektite, and content of alkali (R₂O) is more in obsidian. Tektite is formed under ultra-high temperature and pressure, and it shows highly non-crystallizing and disordering. Stretching vibration mode of [SiO₄ ] appears at 1 200~900 cm^-1 in tektite and obsidian. Comparing with tektite, obsidian is formed under lower temperature and pressure. The absorptions at 750~500 cm^-1and 1 170 cm^-1 indicate the existence of quartz and plagioclase. In conclusion, different forming condition of obsidian and tektite caused differences in chemical compositions and infrared absorption characteristics. Therefore.these two kinds of natural glass can be identified in daily gem inspection.