AVASPEC 2048: AN INNOVATIVE SPECTROSCOPIC METHODOLOGY TO DISCRIMINATE THE QUALITY GEM MINERALS

The AvaSpec 2048 is a spectrometer that allows to obtain the electromagnetic spectra in the  range between 400 and 1000 nm, i.e. in the wave band of the visible and close to the infrared. The spectrometer is equipped with an Avasoft software that convert the AvaSpec signals into absorption spectra. The absorption mode (or transmission mode) wavenumbers can be read by means of a USB port, reproduced on a spreadsheet and processed. At lower wavenumber the spectra are characterized by a strong background noise (from 350 to 430 nm) produced by the weak light source power. To solve this problem, the spectra were reproduced and cleaned of the noise, with the statistical software IGOR Pro. The analyses have been carried out on a large number of green minerals that also represent the most important natural and synthetic gemstones present today on the market: emeralds, tourmalines, garnets, opals, quartz, corundum. The aim of this work is to provide an original database for the gem-quality green minerals. All the samples have been previously investigated to determine the common gemmological properties (size, gravity measurement, colour, luster, cut, fluorescence) and subsequently analyzed through the AvaSpec 2048 equipment. We collected spectra of gemstones characterised by the same chromophore chemical elements in different legand field [2]: for example Cr, Fe, V responsible of the different shades of colour. We also collected several spectra on minerals like beryl aquamarine and zoisite tanzanite [1], that are precious gemstones frequently submitted to thermal treatment aimed to the improvement of the colour. These ameliorative treatments are very commonly carried out on the gems before the marketing. The spectra obtained on the minerals before and after the thermal treatment show several and relevant differences, and therefore represent a useful and non destructive method to recognize the colour change that take place through the heating. We also investigated some synthetic Beryl Emeralds (AGEE, Biron) and artificial material: it resulted that the spectra are very different and easily recognizable from the natural ones.