gems, jade, nephrite, China, Chinese jade carving, Guangxi

Twelve black nephrite samples collected from the Guangxi region of southern China were investigated by standard gemological testing, polarized microscopy, scanning electron microscopy, and electron microprobe analysis, as well as infrared, Raman, and ultraviolet/visible/near-infrared spectroscopy. Originating from Ca-skarn contact metasomatism between limestone bearing siliceous rock and diabase intrusions, black nephrite from Guangxi consists mainly of actinolite or ferro-actinolite and minor stilpnomelane, andradite, apatite, epidote, quartz, diopside, pyrrhotite, and pyrite. Its high refractive index (1.625–1.650) and specific gravity (3.015–3.405), as well as its black color in natural light for plate samples and greenish yellow/brownish yellow or pale green/yellowish green/green color in transmitted light for thin-section samples, are primarily attributable to a high iron content: 11.67–25.75 wt.% Fe oxides and a Mg / (Mg + Fe²⁺) ratio of 0.765–0.343. Mid- and near-infrared spectra and Raman spectra are characterized by four vibration bands corresponding to hydroxyl groups coordinated to three cations in M1 and M3 positions for (M M M)OH combinations: (Mg)OH, (MgFe²⁺)OH, (MgFe²⁺)OH, and (Fe²⁺)OH. Their relative intensities further indicate different iron contents and isomorphous substitution of Fe²⁺ for Mg²⁺ ions.