Micro-Raman investigations on inclusions of unusual habit in a commercial tanzanite gemstone

The vibrational properties of sub-millimetre size inclusions of unusual habit in a commercial tanzanite gemstone were investigated by confocal Raman micro-spectroscopy with the aim of probing both their chemical composition and crystal structure. Highly contrasted Raman spectra were recorded in confocal conditions from several inclusions incorporated at different depths, ranging between few microns to some tens of microns beneath the gemstone surface. The observed spectral features were identified as specific markers of hematite (-Fe2O3). Their unambiguous assignment has been inferred by comparing our experimental findings with the literature data recorded either in single crystals of hematite or in other iron oxides and oxyhydroxides. Our results rule out the presence of any pseudomorphic variety of hematite in the investigated gemstone, while confocal micro-Raman spectroscopy definitively proved itself as a very reliable, relatively costless and non-invasive tool for unambiguous identification of sub-surface regions of gemstones.