Gemological Characteristics of Lvwen Stone and Its Color Genesis

Abstract

“Lvwen stone” is a yellow-green carbonate jade gemstone. In this study, the gemological characteristics and color genesis of Lvwen stone were investigated using conventional gemological testing methods and analytical techniques, such as X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), ultraviolet–visible spectroscopy (UV–VIS), laser ablation plasma mass spectrometry (LA-ICP-MS), and scanning electron microscopy (SEM). The chemical composition of Lvwen stone is mainly Ca, with lesser amounts of Mg, Mn, Cu, Zn, Fe, and other trace elements. The rare earth element distribution pattern indicates that Lvwen stone is characterized by MREE depletion and a positive Ce anomaly. The mineralogic composition of Lvwen stone is calcite, and trace-element- and crystal-size-induced colors result in its characteristic banded appearance. The white (or light green) bands consist of comparatively coarse calcite crystals (~100 μm) that are oriented perpendicularly to the band plane, accounting for their poor light transmittance. In contrast, the dark green matrix is composed of cryptocrystalline calcite crystals that are uniform in size (~10 μm) and tightly packed, resulting in superior light transmittance. Lvwen stone has a 6A1→4E(4D)d-d intra-ion electronic transition absorption band of Fe3+ at ~380–450 nm and a 2E→2T2(2D)d-d intra-ion electronic transition absorption band of Cu2+ at ~580–780 nm. This indicates that both the intra-ion electronic transitions of Fe3+ and Cu2+ give rise to the unique yellow-green color of the material. Lvwen stone is produced by ultra-high-pressure tectonic fluids in a relatively closed, reducing environment, and the green matrix was formed earlier than the white bands.