Spectral Characteristics and Coloration Mechanism of Natural and Synthetic Amethyst

In the present work, several spectroscopy analytic methods are applied for testing natural and synthetic amethyst samples, including X-ray fluorescence spectroscopy(XRF), ultraviolet-visible spectroscopy (UV-Vis), FT-infrared spectroscopy(FT-IR) and Raman spectroscopy(RM). XRF analysis results exhibit that the components of both natural and synthetic amethyst samples contain 99% silica and impurities, such as coloration element Fe and other trace metal elements. FT-IR spectra show that natural amethyst samples have a strong and broad absorption band in the 3 560-3 200 cm^-1 range, while synthetic amethyst samples have two sharp water absorption bands, which locate in 3 583 cm^-1 and 3 421 cm^-1, respectively. RM spectra of the natural amethyst sample show the lack of 401 cm^-1 and 1 161 cm^-1 Raman shift. UV-Vis spectra show that coloration of natural and synthetic amethyst samples has the same mechanism, both caused by hole color centers, corresponding to 350 nm and 540 nm absorption bands due to the electronic transition of t1→t*2 and t1→a*,respectively. Heat treatment experiments at different temperatures in the air show that the color centers of amethyst disappear near 400 ℃.