Authentication Of Gemstones Using Raman Spectroscopy

INTRODUCTION

Gemology is a complex field where many disciplines converge: natural sciences (geology, chemistry, physics, mineralogy and crystallography), art, history and archaeology. Economic aspects also play an important role in the study of gems. One of the most important issues in gemological research is the classification of a gem, in terms of mineral species, purity, provenance and identification of enhancement treatments. As a result, Raman spectroscopy is an ideal method for the examination of marketable gemstones because of the lack of sample preparation involved and the nondestructive nature of Raman analysis. Furthermore, Raman spectroscopy offers many advantages, such as short measurement times and the low amount of material required. For gemological purposes, it is particularly appreciated, being a completely noninvasive, contactless technique which does not require any sample preparation.

METHOD

Raman spectroscopy is based on the inelastic scattering of light by polarizable molecules, revealing the vibrational energy levels of the chemical bonds of molecules. The fingerprints of spectra are used to identify counterfeits that are created by Raman spectroscopy, these peaks are associated with the chemical composition of gemstones, as well as the trace minerals and inclusions that give gemstones like emerald and ruby their unique colors.

The standard way to give an answer by means of Raman spectroscopy is by comparison of the spectral fingerprint of the gem with some spectra of standard minerals. Figure 1 shows the Raman spectra of some usual and unusual gems. Luckily, the availability of a large database of Raman spectra of mineral species is constantly increasing in time, so this question is often easy to answer using Raman spectroscopy. Many gems are well-defined mineral species, such as diamond (C); most gemstones are silicates, such as beryl, topaz and zircon, whereas the second most represented class is oxides, such as ruby and sapphire. In this case, the mineralogical species of a gem is quite easy to identify, by means of comparison with literature spectra. As previously stated, even though no comprehensive gemstone Raman database has been built yet.

Figure 1 Raman spectra of some usual gems

In many cases, gems belong to a mineral group which constitutes an isomorphic series whose chemical composition changes continuously within a specified range. One of the most important examples of the use of Raman spectroscopy in determining the composition of gems is that of garnets. This isomorph group is usually divided into two series: pyralspite (pyrope, almandine, spessartine) and granites (uvarovite, grossular, andradite). The Raman spectra of garnets in the ugrandites and pyralspite series are shown in Fig. 2, use minimization software and calculate the composition that better reproduces the frequencies measured in the Raman spectrum of the sample.

Figure 2 Raman spectra of garnets in the ugrandite (a) and pyralspite (b) series

The investigation of the origin of gems by means of Raman spectroscopy could be done by obtaining information on the provenance and genesis of gemstones by Raman spectroscopy is the study of solid or fluid inclusions. Inclusions in gemstones are receiving great interest because they are a sort of ID of the gemstone, containing a large amount of information on its origin. What is more, inclusions embedded in the host crystals can be studied by using a confocal micro-Raman spectrometer which enables detection of materials below the external surface of the gem. We can see that figure 3 shows the Raman spectra obtained on a needle-shaped inclusion in a garnet gem. The Raman spectrum of the host garnet allows its identification as a pyrope–almandine type.

Figure 3 Raman spectra of A pyrope–almandine garnet

SYSTEM

For measurements, we used a modular Raman system comprising the QE Pro spectrometer, a 1064 nm laser for Raman excitation

The QE Pro spectrometer from Ocean Insight is a high-performance instrument renowned for its exceptional sensitivity and quantum efficiency, particularly suited for UV to NIR applications (200-1100 nm). Featuring a back-thinned CCD detector and an integrated cooling system, it minimizes thermal noise, ensuring stable performance for extended periods, which is ideal for low-light and fluorescence spectroscopy. Its high dynamic range allows it to handle varying light levels, making it versatile for applications such as Raman spectroscopy, absorbance, and reflectance measurements. While its advanced features come at a higher cost and may require a learning curve, users highly regard the QE Pro for its reliable, high-quality spectral data and compatibility with a range of Ocean Insight accessories and software.

Figure 4 QE Pro Spectrometer

The Ocean Insight Raman Laser 1064 nm is a high-performance, near-infrared (NIR) laser designed specifically for Raman spectroscopy applications. Operating at a wavelength of 1064 nm, it offers the advantage of reduced fluorescence interference, making it ideal for analyzing complex biological and chemical samples. The laser delivers stable and precise output with high spectral purity, ensuring accurate and reproducible Raman measurements. Its robust design and compatibility with Ocean Insight's modular spectroscopic systems allow for seamless integration into various experimental setups. As a result, users appreciate its reliability and efficiency in providing clear Raman signals, though some may find the initial setup requires careful calibration and alignment.

Figure 5 Ocean Insight 1064 nm Raman Lasers

Conclusion

The authentication of gemstones using the Raman QE Pro spectrometer has proven to be highly effective, providing precise and reliable identification based on the unique vibrational signatures of each gemstone. This advanced spectrometer, with its high sensitivity and resolution, allows for non-destructive analysis, ensuring the integrity of the gemstones while delivering accurate results. By comparing the acquired spectra against known reference spectra, the Raman QE Pro spectrometer can distinguish between genuine gemstones and synthetic or imitation materials, making it an invaluable tool for gemologists and jewelers seeking to verify the authenticity of their collections.