Gemmological and Spectroscopic Characteristics of Chrome Tourmaline Appeared on the Market Recently

According to the results of conventional tests, the refractive index of chrome tourmaline samples is ranged from 1.618 to 1.638, and the birefringence is 0. 018 to 0. 020 and the specific gravity is 3. 05. Under the Chelsea colour filter, chrome tourmaline samples do not show red colour, as well as normal green tourmaline samples. All samples present glassy luster, with flawless interior.
Consistent with the standard spectrum of tourmaline, all samples have absorption peaks at 850-700 cm^-1 that attribute to antisymmetric stretching vibration of Si-O-Si in the structure of tourmaline. Peaks at 1 150-980 cm^-1 attribute to vibration of 0-Si-O and peak at 1 333 cm^-1 attributes to vibration of (BO₃)^3-. Peaks at 630-500 cm^-1 attribute to vibration of Si-0.M-0 and (BO₃)^3-. Peaks at 3 900-3 000 cm^-1, got by transmission method, attribute to the vibration of (OH)^-. The molecular structure of chrome tourmaline has no difference with normal green tourmaline according to the infrared spectrum.
UV-Vis spectrophotometer analysis of chrome tourmaline samples presents two strong absorption bands around 400-470 nm and 570-630 nm, and an absorption peak at 680 nm.According to the previous research of UV-Vis spectrophotometer analysis of colour-causing Cr³⁺, the spectrum would be attributed to Cr³⁺. Unlikely, the normal green tourmaline samples show quite different spectrum, which has a weak absorption peak at 500 nm and a band around 670-770 nm. This analysis reveals that the chrome tourmalines is caused by Cr³⁺, which is different from normal green tourmalines. Further more, colour parameters of samples, including D-wavelength, purity, and CIE coordinate system(L^*, a^*, b^*), do not have significance for the identification of chrome tourmaline.
The results of energy dispersive X-ray fluorescence spectra of chrome tourmaline samples show the existence of chromophore as Cr and V, besides of ordinary component elements such as Ca, Mn and Fe, while the normal green tourmaline samples lack Cr and V.
The results of researches suggest that the colour of chrome tourmalines from jewelry market is indeed caused by Cr³⁺, which can be used to distinguish from normal green tourmalines by UV-Vis spectrum and X-ray fluorescence spectrum.