Gemmological Characteristic of "Qinghua" Nephrite from Hetian, Xinjiang

As a unique variety of nephrite, although black nephrite ("Qinghua")has not been classified into the mainstream colour variety of nephrite, with the depletion of nephrite mineral resources in recent years, it has gradually attracted people's attention. Therefore, studying the gemmological and mineralogical characteristics of Hetian black nephrites and improving their quality evaluation system have positive significance for improving market awareness.This paper took Hetian black nephrite ("Qinghua") as the research objects, and the samples are from Hetian district, Xinjiang in China. It mainly analyzed the production, structure, chemical composition, microscopic characteristics and quality identification, and try to classify its varieties. It can be determined as Hetian black nephrite by conventional gemmological observation and polarizing microscopy, and then by infrared and microscopic laser Raman spectroscopy, electron probe analysis (EPMA), X-ray powder diffraction (XRD), and scanning electron microscopy (SEM). EPMA chemical composition analysis shows black nephrite has a ratio of Mg/(Mg+Fe) being more than 0.90 and the main mineral is tremolite. In addition to the colored mineral graphite, Hetian black nephrite sample also contains minor minerals such as dolomite, diopside and chlorite, etc. Its formation condition belongs to contact metasomatic genesis caused by intermediate-acidic intrusive mass (post-magma solution) and magnesite dolomite marble.