Mineralogical and Gemmological Characteristics of Gem-quality Forsterite-Serpentine from North-eastern China

Gem-quality forsterite-serpentine is a translucent-to-opaque gem material from northeastern China that entered the market in 2019 and was not well characterised until now. It ranges from greenish yellow to dark green to black, has a compact texture and exhibits good carving performance. It is mainly composed of forsterite (Fo), serpentine and brucite, and exhibits a metasomatic relict mesh texture in petrographic thin sections. The forsterite shows extremely high MgO contents, with Fo contents of 98.28-99.65% (i.e. nearly end-member forsterite), and it has been partially serpentinised into lizardite and chrysotile. Infrared spectra of the material show a mixture of serpentine and forsterite. Based on Raman spectra, the black forsterite-serpentine rock contains more spinel, magnetite and phlogopite than the greenish yellow material. The mineral composition and geochemical characteristics, coupled with the geological setting, suggest that the forsterite-serpentine is dolomite-related in origin and represents a unique type of serpentine-bearing gem material.