Causes of color in purple- and yellow- quartz

Quartz is a variety of gemstone consisting of SiO₂. The various colors of quartz are caused by Fe as a trace element. This study focused on the investigation of Fe oxidation states affecting colorations of purple- and yellow- quartz by X-ray absorption spectroscopy (XAS) and UV-Vis-NIR spectroscopy. The amethyst (purple- quartz) and citrine (yellow- quartz) were collected as the samples. As the XAS results, there was only Fe³⁺ oxidation state on both purple-and yellow- quartz considering with the absorption energy position at the Fe K-edge compared to the Fe-oxide chemical standards i.e., FeO and Fe₂O₃. However, the UV-Vis-NIR absorption spectra of the purple- and yellow- quartz were different. Therefore, we propose that the coloration of purple- and yellow- quartz was caused by Fe³⁺ oxidation states as the donor states at 3.59 eV and 2.28 eV with the different energy band gaps of the purple- and yellow- quartz that were 4.13 eV and 4.88 eV respectively derived from Tauc Plot method.