UV-Vis-NIR Spectroscopic Characteristics of Vanadium-Rich
Hydrothermal Synthetic Emeralds From Russia
YANG Yan-ling1, Andy Hsitien Shen1, FAN Yu-rong2, HUANG Wei-zhi1, PEI Jing-cheng1*
1. Gemmological Institute，China University of Geosciences (Wuhan)，Wuhan 430074，China
2. College of Life Science and Technology，Huazhong Agricultural University，Wuhan 430072，China
Abstract：Emerald is a species of beryl in which chromium (Cr) and vanadium (V) are co-colored, with a long history of synthesis, continuous technological improvements and the emergence of new formulations. Recently, a new type of synthetic hydrothermal emerald with a bright color whose appearance is comparable to natural emeralds from Colombia has shown up in the market. After initially analyzing the emerald, it was a vanadium color-causing synthetic emerald. In order to investigate its characteristics, a detailed study was carried out using LA-ICP-MS and UV-Vis-NIR spectrophotometer, aiming to obtain the content of chromogenic elements and analyze the chromogenic reason in its chemical composition and UV-Vis-NIR absorption spectra. Digging differentiation from natural emeralds, we could provide vital data for testing institutions. The chemical composition showed that these synthetic emeralds were pure vanadium-colored, characterized by vanadium-rich and iron-poor, with copper (Cu) varying widely among the different batches, while Cr and other color-causing elements were mostly below the detection limit. The conventional iron-rich hydrothermal synthesized emerald samples used for comparison were characterized by chromium-rich and iron-rich. In addition, it contained high nickel (Ni) and traces of titanium (Ti), manganese (Mg) and Cu, while the vanadium content was below the detection limit. The UV-Vis absorption spectra of the new synthetic emeralds revealed typical absorption spectral features of vanadium, with two broad absorption bands centered at 430 nm in the violet region and 617 nm in the orange-red region, in addition to a shoulder peak near about 390 and 680 nm respectively, and a weak absorption peak at 756 nm for most samples. 430 nm absorption band was attributed to the d electrons spin-allowed transition [3T1g(3F)→3T1g(3P)] of V3+ , the 617 nm absorption band was attributed to d electrons spin-allowed leap [3T1g(3F)→3T2g(3F)] of V3+, and the 756 nm absorption peak was due to Cu2+, and this spectral absorption feature was different from that of the conventional iron-rich synthetic emerald. Most natural emeralds have a combination of Fe3+, Fe2+ and Cr3+ absorption spectra, which can be easily distinguished from these synthetic emeralds. A small amount of the pure vanadium-colored natural emeralds also have the characteristic absorption peaks of vanadium, but they can be separated from the vanadium-rich synthetic emeralds because they also have the characteristic absorption band of Fe2+ around 810~830 nm. The NIR region, mainly showing type I water-related absorption peaks at 1 402, 1 467 and 1 895 nm, can also be differed from natural emeralds. UV-Vis-NIR spectroscopy is an effective means of identifying natural emeralds from synthetic emeralds. However, it should be combined with other identification evidence, such as inclusions and molecular vibration spectroscopy techniques, avoiding the appearance of new synthetic formulations of emeralds that could lead to erroneous identification conclusions.
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