Gemmological Characteristic of A Special Colour-Changed Sapphire

The sample in this study is a sapphire with special colour-change effect, which is from Tanzania. The sample weighs 8.86 ct and has a size of 10.59×13.82×6.35 mm. Its appearance shows light yellow under D65 light source (colour temperature 6 500 K) and light purple red under a light source (colour temperature 2 856 K). This study mainly focused on the gemmological characteristics of this sapphire sample, such as inclusions, trace elements, origin of colour and colour-changed effect, fluorescence characteristics, etc. Microscopic observation and photomicrograph, Raman spectrometer, Fourier transform infrared spectrometer, laser ablation inductively coupled plasma mass spectrometry and ultraviolet-visible spectrometer were used to test the sapphire sample. The results show that a large number of flake, needle-like inclusions with metallic luster and colourless transparent columnar crystalline inclusions can be observed under the microscope. They are identified as rutile and zircon respectively by Raman spectroscopy. There are OH peaks of kaolinite in the FTIR spectrum of this sapphire, where contains kaolinite inclusions that is difficult to observe under the microscope. The main trace elements are Cr, V, Ti, Mg, Fe. In the UV-Vis spectrum, there is an absorption band centered at 560 nm, which causes the colour-changed effect. This absorption band is caused by Cr and V. The sample has red fluorescence and red fluorescence is caused by Cr according to the PL spectrum and trace elements.