Gemological and Spectral Characteristics of a New Type of Emerald Synthesized by the Hydrothermal Method

The emerald is loved by people because of its gorgeous color and is respected as one of the world's four major gems. Due to the high value of emeralds, a variety of synthesis and optimization methods have emerged. The hydrothermal method is one of the common methods to produce synthetic emeralds. The emeralds synthesized by this method have typical water ripple texture, showing a serrated parallel arrangement, and the characteristic peaks of the infrared absorption spectrum in the range of 2300-3100cm^-1 are the main identification feature. However, some emeralds synthesized by this method do not have the above typical characteristics. This has brought a great challenge to the quality inspection agencies, and brought many doubts to the majority of consumers, and affected the stability of the gem market.

It is essential to study the new emerald synthesized by this new hydrothermal method. The refractive index, density, polychromatism, and infrared absorption spectrum characteristics of the tested samples are very similar to those of natural emeralds, so infrared absorption spectrum analysis cannot obtain the supporting evidence of its cause. Based on this, the gemological and spectral characteristics of the test samples were studied by ultraviolet visible spectrum, energy dispersive X-ray fluorescence spectrometry (EDXRF) and other spectroscopic means. Finally, the test samples were determined to be a new kind of emerald synthesized by the hydrothermal method.

The refractive index, birefringence, relative density, polychromaticity and weight of the samples were tested by refractometer, electronic balance, and dichroic mirror. The internal and external characteristics of the test samples, the cutting and polishing quality of the samples, and the characteristics of internal inclusions were observed by gem microscope. The infrared reflection spectrum and infrared transmission spectrum characteristics of the samples were analyzed by Fourier transform infrared spectrometer and compared with the infrared spectrum characteristics of emeralds from different origins. The absorption spectrum characteristics of the samples were analyzed by ultraviolet-visible spectrometer, focusing on the absorption peak characteristics near 830nm. The element characteristics of the samples were tested by EDXRF, because the content characteristics of Cr and Ni in emerald are the key to identify whether it is natural or not.

The mass of the tested samples was 0.17-0.56g, the refractive index was 1.571-1.588, the birefringence was 0.007-0.009, the relative density was 2.56-2.78, and the polychromaticity was obviously blue-green/green. These conventional gemological characteristics were consistent with natural emeralds. The samples were observed under a gemstone microscope, and the samples had a parallel texture of suspected water ripple, but not typical. The water ripple texture of the typical hydrothermal synthetic emeralds was serrated parallel arrangement. However, the characteristics under the gem microscope should arouse the suspicion of jewelry inspectors.

The infrared reflectance spectra of the samples had characteristic peaks of 1242, 1219, 1022, 968, 810, 741, 687, 594, 528, 490, and 459cm^-1. The characteristic peaks of 1242, 1219, 1022, 968, 810, 741, and 687cm^-1 were caused by the stretching vibration of Si—O—Si and O—Si—O, and the characteristic peaks of 594, 528, 490, and 459cm^-1 were caused by the bending vibration of Si—O and the vibration of M—O. The infrared reflection spectrum characteristics of the samples were consistent with the infrared spectrum characteristics of beryl, but the infrared spectrum characteristics of the fingerprint area could not indicate whether the samples were natural or synthetic.

The infrared transmission spectra of the samples had 5445, 5273, 5109, 4874, 4800, 4650, 4547, 3321, 3222, 3027, 2953, 2928, and 2431cm^-1 characteristic peaks. 5273cm^-1 was caused by typeⅡ water absorption, 5445cm^-1 and 5109cm^-1 were caused by typeⅠ water absorption. The typeⅡ water absorption peak of the test samples was relatively sharp at 5273cm^-1, and the typeⅠ water absorption peaks at 5445cm^-1 and 5109cm^-1 were relatively strong. Compared with previous studies, the shape and intensity of water absorption peaks of natural emerald and hydrothermally synthesized emerald were different. The infrared absorption spectrum characteristics of the water of the test sample were different from those of natural emerald, but the diagnostic direct evidence could not be obtained from the infrared absorption spectrum. The infrared peak in the range of 2300-3100cm^-1 is caused by Cl^- ion absorption. According to the results of previous studies, the absorption intensity of hydrothermal synthetic emeralds in this wavenumber range was significantly stronger than that of natural emeralds. The infrared absorption spectra of the test samples were relatively weak in the range of 2300-3100cm^-1, which was more similar to the infrared absorption spectrum characteristics of natural emeralds in Colombia, Yunnan, Madagascar and Brazil, and was obviously different from the infrared absorption spectrum results of emeralds synthesized by the hydrothermal method. There were obvious differences between the infrared transmission absorption spectra of the traditional hydrothermal synthetic emeralds and natural emeralds. The characteristics of infrared absorption spectra are important evidence for identifying the genesis of emeralds. However, it is difficult to obtain the final accurate conclusion from the infrared absorption spectrum analysis of the test samples described in this paper, so they have been named 'new' type.

The UV-Visible spectrum characteristics of the test samples showed that the absorption peak below 350nm was caused by the Fe²⁺-O^2- charge transfer band in emerald, the absorption peak at 429nm was caused by Cr³⁺ ions, the broad absorption band at 550-650nm was mainly caused by Cr³⁺ ions, the absorption peak at 683nm was caused by Cr³⁺ ions, the sharp absorption peak at 956nm was caused by water absorption, and the broad absorption peak near 850nm should be caused by Fe²⁺ absorption, which was rarely seen in the previous hydrothermal synthesis of emerald. The combination of Cr³⁺ and Fe³⁺ replacing Al³⁺ makes emerald, green. The absorption peak of UV-Visible spectrum caused by Fe²⁺ in natural emerald was in the range of 810-850nm, and only a few of the absorption bands caused by Fe²⁺ in natural emeralds are not obvious. The absorption band of the UV-Visible spectra of the test samples was not obvious or weak in the range of 810-850nm, and the 850nm absorption peak of individual samples was obvious. Therefore, the emeralds examined by ultraviolet-visible spectroscopy may be synthetic, but this could not be conclusively confirmed.

The EDXRF results showed that the samples had Fe, Ni and Cr elements, which was different from previous studies of natural emeralds. Yang Zhuo(2010) studies showed that the main trace elements in natural emeralds, from different origins such as Brazil, Colombia, Egypt, India, South Africa, Zimbabwe and Canada by LA-ICP-MS, were Fe, V, Cr, Mg, Na and Cs, but the content of Ni was very low, far lower than that of Cr. The content of Cr in Yunnan emeralds was 58-177μg/g, and Ni was not detected (Bai et al., 2019). The chemical element characteristics of gem-grade emeralds in Afghanistan, Brazil, Colombia, Ethiopia, Madagascar, Russia, Zambia and Zimbabwe showed that Ni element was not detected (Karampelas et al., 2019). The elemental composition of emeralds from South Africa, Afghanistan, Australia, Colombia, Egypt, Madagascar, Zambia, Zimbabwe, Tanzania and China was analyzed by electron probe microanalysis, and Ni was not detected (Lum et al., 2016). It could be seen that Cr and Fe elements coexisted in natural emeralds, but Ni was almost non-existent. EDXRF was used in this study without quantitative analysis but calibrated by the gold standard sample, and the relative peak intensity of the element was of great significance. The peak intensity of the Ni element in the test sample was much higher than that of the Cr element, which did not conform to the element content characteristics of the natural emerald, and was consistent with the high content of Ni in the hydrothermal synthesis of emerald reported by Lu and Shen (2021) and Yang et al.(2022). The comprehensive analysis showed that the test sample was a synthetic emerald.

The infrared absorption spectrum characteristics of the test samples are similar to those of natural emeralds, and the infrared absorption spectrum could not be used as a basis for identifying the genesis of emeralds. Previous studies show that a set of absorption peaks in the range of 2300-3100cm^-1 in the infrared absorption spectrum is an important identification feature of the hydrothermal synthesis of emeralds, but this feature is not obvious in the tested samples. The UV-Visible spectrum of the sample shows that the absorption band near 810-850nm related to Fe²⁺ is not obvious. The X-ray fluorescence spectra of the samples show that the Kα peak intensity of Ni is much higher than that Kα of Cr, which is the main feature of the new emerald synthesized by the hydrothermal method. It could be seen from the experimental results that the identification of jewelry and jade is a systematic work. It is necessary to learn from the previous results and combine the comprehensive analysis of various detection methods to obtain the correct result.