Characterization of emeralds by micro-Raman spectroscopy

In recent years the use of Raman spectroscopy as a gemological tool has largely increased. In particular, in the conservation science field, the possibility to have a quick, non-destructive, contactless identification of a gem, maybe mounted on precious archaeological item, made this technique an invaluable procedure for gemologists and conservators. The results of the Raman analysis are not limited to the simple identification of a gem. In this work we show the large amount of information which is possible to obtain on one of the most important gems, emerald, the green variety of beryl. We studied by means of a standard micro-Raman spectrometer a large group of emeralds in different forms and of different origin: 15 faceted gems and a series of raw crystals (some of them still embedded in the host rock) coming from Val Vigezzo (Western Alps). Some fakes have been identified between the faceted gems (a garnet, a glass, a “quartz-beryl” sandwich). All the natural gems and crystals have been fully characterized from the vibrational point of view. In particular, the high frequency spectrum, in the OH-rich region, was used to estimate the amount of alkali ions present in the channels of the structure [Łodziński et al., 2005]. To quantify Be and Li, alkali ions in the channels such as Cs, Rb, K, Na, and other elements which better define the structure of beryl such as V, Cr, Mn, Fe analyses with LA-ICP-MS have been performed. Fig.1: Raman spectra of two simulants found between the faceted gems and the characteristic Cr3+ luminescence of the emerald excited at 473.1 nm. In addition, solid inclusion were identified and used as a tool to differentiate the provenance of the emeralds. The shape and the position of the characteristic laser-induced luminescence of chromium ions was used to better define the origin of the gems [Moroz et al., 2000]. Fluid inclusions present in the alpine crystals were studied; the identification of the phases and their concentration obtained by micro-Raman spectroscopy was completed by thermal analysis in order to made hypothesis on their genesis. Fig.2: Raman spectra of an emerald coming from Val Vigezzo and of the liquid and gas phases present in a fluid inclusion; the broad band of liquid water, the sharpest one of the channel water at ~3600 cm-1 and the peak of methane at 2915 cm-1 are clearly visible. References Łodziński M., Sitarz M., Stec K., Kozanecki M., Fojud Z., Jurga S. Journal of Molecular Structure 2005; 744–747: 1005–1015 Moroz I., Roth M., Boudeulle M., Panczer G. Journal of Raman Spectroscopy 2000; 31: 485-490