The Color Quantitatively Expression, Rehabilitation and Color origin study of gray-purple nephrite from Qinghai, China

Abstract:

Gray-purple nephrite is a special type of nephrite occurred in Sanchahe mining, Qinghai province, China. To quantitatively study its color, a representative gray-purple nephrite with typical color change from light to dark color was chosen as the experiment sample. By combination of the UV-VIS transmittance spectra and 1976 CIE L*a*b* Colorimetric parameters, we first realized the quantitative expression and color rehabilitation of the transmitted light color of this nephrite with a specific thickness (1.0 mm). Besides, according to the typical emission peak centered at 580 nm of photoluminescence spectra, and the absorption peak centered at 530 nm, as well as the contents of Mn element increased with the purple becoming darker, we can inferred that the purple color of nephrite was mainly caused by Mn²⁺. This conclusion was confirmed by our EPR spectroscopy results. For the famous color pattern “stream line” in the dark region, shown as color stripes with 100~10 μm size, we will also use micro-XRD mapping and in the future XANES mapping to determine the phase and chemical composition. This research method could offer valuable references to the color quantitatively expression and coloration mechanism research of other jewelries.