Abstract: In March 2018, two "tanzanites" were bought at a low price from Shuibei jewelry market in Shenzhen. The samples were found to be coated topaz through the conventional detection. Then a series of tests were proceeded, such as infrared spectroscopy, X-ray fluorescence spectrometer, UV-Vis spectra and so on. One of the samples was poliched and a colorless topaz was came to light, which proved that the bottom of the sample was coated by colored materials. Chemical analysis suggested that the main material of the coated layer contained titanium and the content ranges from 2.769% to 5.039%.UV-Vis spectra test showed 326, 531 nm absorption peak. The purpose of coated topaz was to change its colour to simulate tanzanite.