Gemmological Characteristic of A Colour-Changed Sapphire from Tanzania

The sample in this study is a sapphire with special colour-chang effect produced in Umba, Tanzania. It shows pale yellow under D65 light source, light purple-red under light source A, and red fluorescence under long-wave UV light (365 nm). In order to study the gemmological characteristics of this sample, including inclusions, trace elements, colour and colour-chang effect, fluorescence characteristics and the other gemmological properties, this sample was tested by microscopic observation and photographing, Raman spectrometer, FTIR spectrometer, laser ablation inductively coupled plasma mass spectrometer and UV-Vis spectrometer. It was found that there are needle-like rutile inclusions, cluster zircon inclusions and very small kaolinite inclusions. The trace elements in the sample are Cr, V, Ti, Mg, Fe, etc. The wide absorption band centered at 560 nm in visible region causes the colour-chang effect. Based on the charge compensation theory of sapphire and contents of trace elements, this absorption band is mainly caused by Cr and V. According to the photoluminescence spectrum and trace elements, the red fluorescence of the sample is produced by Cr.