Characteristics and Identification of the Skin Colour of White Hetian Yu Pebbles from Jade Deposit No.1 in Hotan Prefecture and Imitation Pebbles

Modern analytical methods including FTIR, UV-Vis spectroscopy, Raman spectroscopy and LA-ICP-MS were adopted to conduct a comparative study on the skin colour characteristics of natural white Hetian Yu pebbles mined from the jade deposit No.1 in Hotan Prefecture, Xinjiang, China and artificially dyed imitation pebbles collected from the same mining area. The results show that natural white Hetian Yu pebbles feature diverse skin colours with natural gradient transitions and distinct layering, while imitation pebbles present a single, vivid hue confined to the surface, with observable dye deposits. Infrared spectoscopy indicates that natural white Hetian Yu pebbles exhibit typical spectra of tremolite, with only slight differences in peak intensity among different skin colours. Although the infrared spectra of imitation pebble samples retain the main tremolite peaks, C-H stretching vibration peaks of organic dyes appear in the 2 800-3 000 cm^-1 range. UV-Vis absorption spectroscopy demonstrates that natural Hetian Yu pebble samples have characteristic peaks near 280 nm and 580 nm as well as around 730-750 nm, whereas dyed imitation pebble samples show a strong characteristic peak in the 445-460 nm range, without the characteristic broad peaks of natural pebbles. Raman spectroscopy confirms that tremolite is the dominant component of all samples in this study. However, for the skin of imitation pebbles, a broad and gentle hump appears in the 100-1 800 cm^-1 range, accompanied by multiple characteristic peaks of organic dyes. LA-ICP-MS analysis reveals that the major element compositions of natural pebbles are consistent with the theoretical composition of tremolite. The trace element compositions between the skin and internal matrix of natural samples are continuous without significant enrichment, showing a flat rare earth element distribution pattern and a significant negative Eu anomaly. In contrast, imitation pebbles have a relatively higher mass fraction of K₂O, with abnormal enrichment of elements including Mn, Fe, Zn, Sr and Ba in the skin, forming an obvious element discontinuity with the internal matrix; the total REE content of imitation pebbles is higher, with a relatively disordered distribution pattern. Based on the comparative results of skin colour observation, spectral characteristics, and compositional analysis, this study summarizes the differences and identification features between the skin colour of natural Hetian Yu pebbles and dyed imitation pebbles.