Gemmological and Spectroscopic Characteristics of Synthetic Alexandrite by the Horizontally Oriented Crystallisation Method

Cumently, the market is dominated by synthetic alexandrite synthesized by the czochralski method, while the synthetic alexandrite synthesized by the Horizontally Oriented Crystallisation (HOC) method is relatively rare, and the study of its gemmological and spectroscopic characteristics still needs to be supplemented. In this paper, 8 synthetic alexandrite by HOC method from a Russian manufacturer were selected as the research objects and tested and analyzed using conventional gemmological instruments such as refractometers, laser-ablation inductively coupled plasma mass spectrometer (LA-ICP-MS), energy dispersive X-ray fluorescence spectrometer (EDXRF), UV-Vis spectrometer, fluorescence spectrometer, Raman spectrometer, and Fourier transform infrared (FTIR) spectrometer, to explore its inclusion characteristics, trace elements compositions, infrared spectrum, UV-Vis spectrum, fluorescence spectrum, and other characteristics to dirstinguish it from natural alexandrite. The magnified observation results show that the typical inclusions of the synthetic alexandrite samples by HOC method are a large number of oriented elongated voids and clustered metal inclusions. Chemical composition tests show the presence of the colour-causing elements Cr and V. The Cr content is higher than that of natural alexandrite. Fe element content is very low; the content of Mg, Ti, Ga and other elements are lower than the natural alexandrite, with unusual high Mo content, presumably due to the Mo crucible residue. UV-Vis spectrometer test results show a typical chromium spectrum, with two broad absorption bands in the orange-yellow and violet regions, shoulder peaks at 645 nm and 656 nm, and a weak absorption peak at 680 nm. Three-dimensional fluorescence test concluded that HOC method synthetic alexandrite has strong fluorescence peaks at 678 nm and 680 nm, and weak fluorescence peaks at 690 nm and 696 nm, which are all caused by Cr element, and does not have the luminescence center at 460-550 nm caused by Ti and O element in natural alexandrite. The infrared absorption spectrum shows that it does not have the absorption peaks at 2 160 cm^-1 and 2 402 cm^-1, which are unique to natural alexandrite, and there is no obvious water related absorption between 3 000-3 500 cm^-1.