Gemmological Characteristic of Brown Sapphire from Guinea, Africa

The gemmological characteristics of sapphire from Guinea, Africa were investigated by laser Raman spectrometer, Fourier transform infrared spectrometer(FTIR), laser ablation inductively coupled plasma mass spectrometer(LA-ICP-MS) and micro-ultraviolet-visible spectrometer(UV-Vis). The analytical results show that sapphires from Guinea, Africa are mainly brown, with a minor contribution of green colour. After polishing into 0.4 mm slices, they show a reddish brown colour with obvious colour bands. Magnified observation showed that there are a lot of inclusions in sapphire and most of them are needle-like inclusions. Combined with Raman spectrum test results, it shows diaspore, rutile, pyrite, magnetite, hematite, anatase and other inclusions. The infrared spectra show obvious O-H vibration peaks related to diaspore at 1 990, 2 123, 2 912 cm^-1and 3 033 cm^-1, indicating that the sapphires were not heated. The contents of Fe, Ti and Cr in brown sapphires from Guinea are high. Fe, Ti and other trace elements between dark and light colour bands are quite different, which may be related to the enrichment of many inclusions in the dark colour band. According to the Fe/Ti ration(more than 10) in the light colour zone, brown sapphires from Guinea can be inferred as basaltic type. The UV-Vis absorption spectra show 377, 387 nm and 450 nm absorption peaks caused by Fe³⁺, and 556 nm absorption broad band and 693 nm absorption sharp front related to Cr³⁺ ions. The red body colour of brown sapphire is mainly caused by Cr³⁺, and the overall brown colour is caused by a large number of brown inclusions in the interior.