Gemmological Characteristic of Electron Irradiated Amethyst

Amethyst from East Africa and ametrine from Bolivia and laboratory were all irradiated under electron beam and then measured by micro UV-Vis spectrometer and MIR spectrometer. The absorption spectra have been studied to analyze the change after the treatment and to explain the colour mechanism in some degree. Treatment experiments showed that the electron irradiation can intensify the violet colour effectively only in specific colour zones of amethyst with the phenomenon that the absorption band centered at 540 nm gets higher. The radiation induced the peak shift from 221 nm to 228 nm in ultraviolet region of amethysts from East Africa, and the peak shift from 3 614 cm^-1 to 3 620-3 623 cm^-1 in middle infrared region of all the amethysts, which may have significance on identification of the treated amethyst. The irradiation treatment on heated ametrine made the completely faded sample's violet reverse, and the violet concentrated along Brewster fringes. This experiment confirmed further that the Fe in amethyst has strongly directional selectivity that supports the viewpoint where the occupations of Fe do matter on coloration. Also, we found detectable structural water bands in MIR region have no obvious correlation with colour.