Comparative study of emeralds from different geological occurrences

Abstract

Thailand is known as one of an important center for precious stones cutting, jewelry manufacturing and exporting. Rough or cutting emeralds have been imported to Thailand from foreign countries especially from Colombia, Brazil, Zambia, Madagascar and Nigeria. Determination of origin of emerald can be useful as value added factors in the gem marketing. This study aims at comparing the gemmological properties, internal characteristic, absorption spectra and chemical composition of emeralds from the different geological occurrences. Totally 149 emerald samples from various types of important geological occurrence were acquired for this study. They can be divided into two different major categories: non-schist-related type I deposits (Nigeria and Colombia) and schist-related type II deposits (Carnaiba/Socoto, Itabira and Santa Terezinha in Brazil, Madagascar and Zambia). Some characteristics are locality specific and can be used for origin determination.In addition, refractive index (RI) and specific gravity (SG) are related to alkali contents in emerald. The high to medium RI and SG values are found in emeralds from type II deposit which is high in alkali contents. In contrast the low RI and SG values are found in emeralds from type I deposit which is low in alkali contents. The emeralds from Zambia and Carnaiba/Socoto contain relatively high contents of Li and Cs, while the emeralds from Madagascar show exceptionally high K content. The Colombian emeralds show consistently very low iron content. Some inclusion features can be used to distinguish emeralds from different locality such as emeralds from Colombia and Nigeria contain different jagged outline in 3-phase inclusions in which those from Colombia have lower liquid/vapor ratios. For the spectroscopic properties, UV-Vis-NIR absorption spectra of emeralds from most localities show a combination of Cr[superscript 3+]/V[superscript 3+] and Fe[superscript 2+,3+], except the spectrum of Colombian emerald has no Fe[superscript 2+,3+] absorptions. NIR absorption spectra of water molecules (type I and II) in emerald{7f2019}s structure show relationship to alkali contents. In addition, emeralds from Nigeria of type I deposit is dominated by the type I water. In contrast, emeralds from Santa Terezinha in Brazil of type II deposit is dominated by the type II water