Abstract:
Tourmaline, as a kind of beautiful colorful gemstone, is increasingly popular in our life. The price of gemstones is mainly limited by clarity, color, and transparency. The output of high-quality natural tourmalines is getting less and less, thus, finding an alternative way to address the resultant imbalance between supply and demand is paramount. Heat treatment, as a kind of common way to improve the appearance of gemstones, could improve the unattractive color of tourmalines and increase its aesthetic appearance, thereby increasing the prices of tourmalines. Among the different colors of tourmalines, dull yellow-green is an unfavorable color compared with the others. In this paper, we proposed an effective heat treatment approach which can turn such unfavorable color into the beautiful pink, resembling the natural ones. To explore the influences of element type and valence state on the colors of the yellow-green and pink tourmaline and analyze their difference, the tourmaline samples before and after heat treatment were tested by LIBS, XRF and XPS, together with spectra tests like infrared reflectance, ultraviolet reflectance spectroscopy and Raman spectra. On the basis of these results, we further analyzed the mechanism of coloration during heat treatment. The results suggest that the color-causing ions of Afghanistan’s yellow-green tourmalines are manganese and iron, while the color-causing ion of pink ones from the same area is manganese. Finally, suggestions for identifying the difference between natural pink tourmalines and manmade pink tourmalines led by heat treatment are given.