Red gemstones characterization by using micro-Raman spectroscopy. Identification, genesis and treatments in rubies and their imitations

In the framework of a wide project aimed to study the potential of Raman spectroscopy in characterizing gems (1,2) jewels and jewellery collections (3), in this work the results of a micro-Raman investigation on red gemstones are proposed. The use of Raman spectroscopy as gemological tool is recently largely increased, thanks to the possibility to carry out non-invasive, non destructive, quickly and contactless measurements on precious objects and artifacts(4); these requirements are fundamental for gems characterizations, in particular in the case of gems set in jewels and preserved in Museums. In fact, gemstones have been widely used in antiquity toadorn precious objects, such as royal insignia or liturgical objects, often having really high cultural valence. In this specific case, the use of portable equipment is mandatory(5). Otherwise, in the case of loose gems, the possibility to analyse them with a laboratory set-up allows to go deeper inside the characterization of materials, permitting the investigation not only of the mineralogical identity but also the study of inclusions and treatments. The latter aspect represents a crucial feature in the gemmological characterization of gems in a trade context, considering the diffusion of “allowed” and “not-allowed” treatments. In addition, the possibility to identify specific solid or fluid inclusions gives back information on genesis and, sometimes, provenance of gemstones(6), supporting the gemmological attribution and solve identification problems of uncertified materials. For aforementioned, considering the excellent results obtained on blue and green gems, a set of gems supplied by a jeweller as representative of the actual trade of the current red gems trade, have been analysed by using micro-Raman instrumentations equipped with different laser excitation sources. In detail, Raman spectra have been collected on sixteen samples of red loose gems (Table 1; Figure 1) by means a confocal Jobin Yvon Horiba Labram, equipped with the 473.1 nm excitation lines and a micro-Raman Jasco NRS-3100 apparatus, equipped with laser excitation source at 785 nm. The identification of the gemstone has been supported by the comparison between the experimental Raman spectra and reference data from various databases(7,8) and literature(9) .The results allow not only to discriminate rubies from simulant and fakes but also to achieve useful information on inclusions and treatments, obtaining a complete characterization of the studied red gems. [1] B. Bersani, G. Azzi, E. Lambruschi, G. Barone, P. Mazzoleni, S. Raneri, U. Longobardo, P.P. Lottici, J.Raman Spectrosc.,2014, 45, 1293. [2] G. Barone, D. Bersani, V. Crupi, F. Longo, U. Longobardo, P.P. Lottici, I. Aliatis, D. Majolino, P. Mazzoleni, S. Raneri, V. Venuti, J. Raman Spectrosc., 2014, 45, 1309. [3] G. Barone, D. Bersani, J. Jehlička, P.P. Lottici, P. Mazzoleni, S. Raneri,P. Vandenabeele, C. Di Giacomo, G. Larinà, J.Raman Spectrosc.2014, DOI: 10.1002/jrs.4649. [4] D.Bersani, P.P. Lottici, Anal. Bioanal. Chem.2010, 397, 2631. [5] P. Vandenabeele,J. Raman Spectrosc.2004, 35, 607. [6] M. L. Frezzotti, F. Tecce, A. Casagli, J.Geochem.Explor.2012, 112,1. [7] RRUFF Project, Department of Geosciences, University of Arizona, Tucson, USA. http://rruff.info/. [accessed01 April 2015] [8] Handbook of Minerals Raman spectra, Laboratoire de géologie de Lyon ENS, Lyon, France. http://ens-lyon.fr/LST/Raman [accessed 01 April 2015] [9] Minerals Raman Database, Physics Department, University of Parma, Italy. http://www.fis.unipr.it/phevix/ramandb.php