Identifying gemstones is not a trivial task. While properties such as color and transparency can aid in identification, gemstones such as rubies, spinels, and garnets can be difficult to tell apart from these properties alone.
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Many gemologists supplement their visual inspection of gemstones with a range of optical techniques, including refractive index, birefringence, and even spectroscopic measurements. For example, due to the arrangement of atoms in the underlying crystal structure, most garnets have little or no birefringence.
This is not the case for rubies, which have much higher degrees of birefringence and give rise to their distinct pleochroism – this means multiple colors will appear in the stone at any given viewing angle.
What defines a gemstone is the chemical composition, the crystal lattice – the way the individual atoms are arranged within the atomic framework of the gemstone – and sometimes also the habit or typical location where they are found. Identification of chemical composition and binding arrangements requires analytical tools sensitive to the presence of particular elements and their local binding environments.
A spectroscopic tool sensitive to chemical environments, binding forces and, indirectly, elemental information is Raman spectroscopy. Raman spectroscopy can be used to retrieve quantitative and qualitative information about solid and molecular species.
In the jewelry industry, Raman spectroscopy can bring greater confidence in the identification of gemstones. It is a non-invasive technique that requires no sample preparation and can be used on both freestanding or mounted gemstones, making it very flexible for gemstone analysis.
In a Raman experiment, the sample of interest is excited with a given wavelength of light, often using a laser source. The inherently weak nature of Raman signals due to the inefficiency of the scattering process involved means that often relatively high power sources are required.
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Once the sample is excited, the scattered radiation is detected using a spectrometer and the relative energy shift of the observed peaks is used to calculate the frequencies of the different vibrational modes in the sample.
Once the Raman spectrum has been reconstructed, the shapes and positions of the peaks can be used to identify the atoms likely to be present in the sample.
Indeed, the frequency of a vibrational mode depends on the masses of the atoms involved and the strength of the bond between them. In different crystal lattice structures, the bond strengths between atoms in various lattice positions can also vary, so information about the possible crystal structure can also be retrieved.
In addition to gemstone identification, Raman spectroscopy is a powerful forensic tool for the investigation of fake stones. Raman microscopy which provides spatially resolved Raman spectra is particularly useful for the study of defects or potential counterfeits, as it can identify regions where stones have been “treated” with other chemical species to improve their perceived value.
Many of these treatments are almost impossible to detect with the naked eye, and although any treatment a gemstone has undergone must be declared, they are often not intended to claim a higher price. Raman microscopy can identify regions where dopants have been introduced and also help distinguish their chemical structure and identity.
What helps improve the certainty of identification with Raman is a large number of peaks in the spectrum. Molecules A will have 3N-6 vibrational modes, where N is the number of atoms, and although not all of them are active in Raman, a pattern of frequencies can be fitted to give a ‘fingerprint’ of the particular gemstone.
Raman in art
Recent work has developed more portable handheld devices that allow gemstones to be studied in situ as well as the use of mineral Raman frequency databases to aid in positive identification.
With Raman spectroscopy methods capable of differentiating between natural and treated gemstones as well as providing reliable and positive identification of gemstone types, there are now relatively simple procedures to apply Raman spectroscopy for analytical purposes, even on low power portable instruments.
Such developments are particularly beneficial for art history applications because many objects are too fragile to be transported to the laboratory and therefore must be measured in situ.
One of the challenges of Raman spectroscopy is overcoming the fluorescence background that can obscure the vibrational modes of the species of interest from being seen. Choosing staggered excitation wavelengths can be a way to overcome this without compromising the reduced scattering signal from longer wavelength excitation.
This can be particularly useful for the analysis of art where it is likely that there are traces of pigment contaminants and other fluorescent species present on the gemstone.
References and further reading
Shigley, JE (2008). A review of current challenges for gemstone identification. Geology, (64). http://mokslozurnalai.lmaleidykla.lt/publ/1392-110X/2008/4/227-236.pdf
Antao, S. (2013). The birefringent garnet mystery: is the symmetry less than cubic? Powder Diffraction, 28(4), 281-288. doi:10.1017/S0885715613000523
Bersani, D., & Lottici, PP (2010). Applications of Raman spectroscopy to gemology. Anal Bioanal Chem, 397, 2631–2646. https://doi.org/10.1007/s00216-010-3700-1
Elboux, D., Izumi, CMS, & Faria, DLA De. (2016). False turquoises studied by Raman microscopy. International Forensic Science, 262, 196–200. https://doi.org/10.1016/j.forsciint.2016.03.041
Culka, A., & Jehlicka, J. (2019). Gemstone Identification Using a Handheld Offset Sequential Excitation Raman Spectrometer and the RRUFF Online Database: A Proof of Concept Study. EUR. Phys. J.Plus, 134, 130 https://link.springer.com/article/10.1140/epjp/i2019-12596-y