Chemical Controls on the Green Coloration of the Novel Gem Quartzite “Feizhoucui”: A CIE L*a*b* Colorimetric Study

This study investigates the mineralogical composition, color origin, and chromatic classification of “Feizhoucui”, a distinctive green quartzite. Analyses of 54 samples via EPMA, UV-Vis spectroscopy, and colorimetry revealed that its characteristic color is primarily attributed to barian–chromian muscovite, occurring as vein-like or spotted associated minerals within a quartz matrix. The chromophoric muscovite’s crystal chemical formula was calculated as (K0.71Na0.05Ba0.20)0.96 (Al1.66Mg0.16Cr0.22Fe0.03Ti0.02)2.09 (Si3.04Al0.96)4O10(OH)2. UV-Vis spectra confirm that the green hue arises from Cr3+ absorption bands at 610–625 nm and 430–460 nm, while Fe content exerts a minor influence by inducing a red shift of the ~518 nm absorption minimum, thereby reducing the hue angle h°. Cr concentration is the dominant factor, correlating positively with chroma C* and negatively with lightness L*. Quartzite crystallinity negatively correlates with chroma C*, indicating that higher defectivity promotes the incorporation of more color-contributing muscovite. Based on K-means clustering of color data, “Feizhoucui” is classified into three commercial grades: Fancy Intense, Fancy Deep, and Fancy.