Jewelry industry

Use of Raman spectroscopy in the jewelry industry

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 distinguish from these properties alone.

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Many gemologists will supplement their visual inspection of gemstones with a variety 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 with rubies, which have much higher degrees of birefringence and give rise to their distinct pleochroism – this means that more than one color will appear in the stone at a given viewing angle.

What defines a gemstone is the chemical composition, the crystal lattice – how the individual atoms are arranged within the gemstone’s atomic setting – and sometimes also the usual or typical location where they are found. Identifying the chemical composition and binding arrangements requires analytical tools sensitive to the presence of particular elements and their local binding environments.

Raman spectroscopy

A spectroscopic tool sensitive to chemical environments, binding forces and, indirectly, elementary information is Raman spectroscopy. Raman spectroscopy can be used to recover quantitative and qualitative information on solid and molecular species.

In the jewelry industry, Raman spectroscopy can provide greater confidence in identifying gemstones. This is a non-invasive technique that does not require 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 relatively high power sources are often required.

raman, raman spectroscopy, spectroscopy, gemstones, gemstone identification, crystal

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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 modes of vibration in the sample.

Once the Raman spectrum is reconstructed, the shapes and positions of the peaks can be used to identify atoms that may 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 that information about the possible crystal structure can also be gathered.

Applications

In addition to the identification of gemstones, Raman spectroscopy is a powerful forensic tool for the investigation of false stones. Raman microscopy which provides spatially resolved Raman spectra is particularly useful for investigating potential defects or 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 that a gemstone has undergone must be declared, they are often not aimed at claiming a higher price tag. Raman microscopy can identify regions where dopants have been introduced and also help distinguish their chemical structure and identity.

What helps to improve the certainty of identification with Raman is a large number of peaks in the spectrum. A molecule will have 3N-6 vibrational modes, where N is the number of atoms, and although not all of them are active on Raman, a pattern of frequencies can be matched to give a “fingerprint” of the particular gemstone. .

Raman in art

Recent work has led to the development of more portable devices for studying gemstones in situ as well as the use of databases of Raman frequencies of minerals to facilitate positive identification.

With Raman spectroscopy methods capable of differentiating between natural or processed 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 moved to the laboratory and therefore must be measured in situ.

One of the challenges of Raman spectroscopy is to overcome the background of fluorescence which can prevent seeing the vibrational modes of the species of interest. The choice of offset excitation wavelengths can be a way to overcome this without compromising the reduced scatter signal from longer wavelength excitation.

This can be particularly useful for art analysis where it is likely that there are traces of contaminants from pigments and other fluorescent species present on the gemstone.

References and further reading

Shigley, JE (2008). A review of the current challenges for identifying gemstones. Geology, (64). http://mokslozurnalai.lmaleidykla.lt/publ/1392-110X/2008/4/227-236.pdf

Antao, S. (2013). The mystery of birefringent garnet: is 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. Forensic Science International, 262, 196-200. https://doi.org/10.1016/j.forsciint.2016.03.041

Culka, A., & Jehlicka, J. (2019). Identification of Gemstones Using a Portable Sequentially Excited Raman Spectrometer and RRUFF Online Database: Proof of Concept Study. EUR. Phys. J. Plus, 134, 130 https://link.springer.com/article/10.1140/epjp/i2019-12596-y

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