Color Genesis and Formation Process of Green Quartzite Jade “Shetaicui” from Inner Mongolia

“Shetaicui” is a distinctive quartzite jade from the Dashetai area of Inner Mongolia. Its green variety has attracted considerable academic attention because of its characteristic color and relatively high economic value, yet its color genesis and formation process remain poorly understood. Here, green “Shetaicui” was examined by petrographic observation, infrared and Raman spectroscopy, ultraviolet–visible (UV–Vis) absorption spectroscopy, colorimetric measurements, X-ray fluorescence spectroscopy (XRF), and electron probe microanalysis (EPMA). These methods were used to characterize its mineral composition and textures and investigate its color genesis and formation process. The results show that the jade is a fine-grained muscovite quartzite, whereas the wall rock is a fine-grained chlorite–quartz–muscovite schist. Coloration is mainly associated with Cr-bearing muscovite. Cr is the principal chromogenic element, whereas Fe contributes to coloration to a lesser extent. Pearson correlation analysis and multiple linear regression show that Cr2O3 is most strongly correlated with a* and C*, whereas FeOT is most strongly correlated with L* and shows a weaker positive correlation with b*. Petrographic textures and mineral assemblages indicate that the jade and its wall rock underwent regional metamorphism broadly consistent with greenschist-facies conditions. Formation proceeded through three stages: sedimentary protolith formation, regional metamorphism, and jade formation during late fluid alteration. During regional metamorphism, quartz-rich sandy horizons formed a Qtz I-dominated pre-jade quartzite, whereas muddy horizons formed the chlorite–quartz–muscovite schist wall rock. Subsequent fluid alteration led to the formation of Cr-bearing muscovite and Qtz II within the pre-jade quartzite. Cr-bearing muscovite imparted the characteristic green coloration, completing the transformation of the pre-jade quartzite into “Shetaicui” jade. These results clarify the correlations between chromogenic elements and colorimetric parameters in green “Shetaicui” and provide petrographic and mineralogical evidence for this three-stage formation process.