Gemological and Mineralogical Characteristics of Emerald from Ethiopia

Abstract

Ethiopia has been gaining attention in recent years as an emerging source of high-quality emerald. Ethiopian emerald samples with different colors ranging from dark green to light green were selected to study the gemological properties, chemical composition, and spectral characteristics. The Ethiopian emeralds were examined using conventional gemological instruments, including X-ray fluorescence spectrometry, LA-ICP-MS, UV/Vis/NIR, infrared spectrometry, and Raman spectrometry, providing a wealth of data and research information related to Ethiopian emeralds. The EDXRF results show that the chemical composition of Ethiopian emeralds is distinctly regional compared to emeralds of Colombian origin, being low in Cr, low in V, and high in Fe. LA-ICP-MS results demonstrate consistent results for Cr (734.34 to 1644.3 ppmw), V (89.61 to 106.61 ppmw), and Fe (4468.04 to 5022.3 ppmw) based on the chemical composition analysis by EDXRF. In addition, the LA-ICP-MS assay revealed that the combination of alkali metals (Li, Na, K, Rb, and Cs) and some trace elements (Sc, V, Cr, and Fe) could distinguish the Ethiopian emeralds from those from other regions. The Ethiopian emerald had absorption of Fe2+, Cr3+, V3+, and Fe3+, and the typical absorption intensity of Fe2+ (around 850 nm) was higher than that of Fe3+ (around 371 nm) in the UV/Vis/NIR spectra. The infrared spectrum of samples indicated that the absorption of type II H2O was higher than type I H2O in the emeralds from Ethiopia, which is consistent with the high content of alkali metals detected by LA-ICP-MS that would lead to an increase in the content of type II H2O. The Raman spectra showed absorption at 410 cm−1, 569 cm−1, 687 cm−1, 995 cm−1, and 1067 cm−1, with an emerald species recognition pattern. The gas–liquid two-phase inclusions of the emerald in this area were mainly CO2 and H2O, and the samples contained typical dark inclusions of magnesium-rich biotite sheets that revealed the tectonic-magmatic-related geological environment in this region.