Identifying gemstones is crucial in studying cultural heritage because of their immense worth. The most valuable gemstones, such as sapphire, ruby, emerald, and diamond, are commonly replaced with cheaper gemstones with the same look, color, or form. It may be difficult to tell the difference between real and fake gemstones without using instrumental techniques, even for a skilled gemologist.
Counterfeiting is very common for these precious gemstones. Gemmologists use several techniques and methods to deal with counterfeiting and differentiate between real and fake gems. Some classical methods used for this purpose are based on refractive index measurement of double refraction or birefringence and specific gravity. Similarly, various instrumental methods are available, allowing tools to identify gemstones even for non-experts.
Instrumental Techniques for Gemstones Identification
The most commonly used instrumental method for the identification of gemstones is Raman spectroscopy. It provides the most accurate results since it can identify the fingerprint of almost every gemstone available.
Moreover, the non-destructive nature of Raman spectroscopy and the ability to analyze in situ makes it a very useful tool. However, in certain circumstances, Raman spectroscopy cannot be used, for example, in places with a geometric hindrance, such as inside a cabinet. The same issue arises while using classical techniques when art jewelry is analyzed and the gemstones are required to be detached from the jeweled frame.
Fourier transform-infrared (FT-IR) spectroscopy
Another technique for identifying gemstones is Fourier Transform-Infrared (FT-IR) spectroscopy. This can be used by exploiting several available configurations, including Attenuated Total Reflection (ATR), reflectance, and absorbance.
UV-visible-NIR absorption spectrophotometry
The color of most of the gemstones is due to the presence of transition metal ions impurities in their structure. These gemstones are known as allochromatic. Similarly, idiochromatic gemstones have chromophores as an essential chemical component.
UV-Visible-NIR absorption spectrophotometry is suitable for colored gemstones with an incomplete set of 3D electrons of transition metal ions since d-d transitions mainly occur in visible regions.
Gemologists also use this technique while exploiting spectral features generated by color centers and charge transfer of colored gemstones.
Despite these advantages, UV-Visible-NIR absorption spectrophotometry has some drawbacks.
In absorption mode, it only works for transparent gemstones.
Moreover, it can only be used for gemstones analyzed in a laboratory. Another disadvantage of this technique is that it is not suitable for pleochroic gems since these exhibit two or three colors upon viewing from different angles or irradiated with different lights.
Another commonly used technique is artificial treatment such as irradiation or heating. This causes color changes due to the creation of color centers or induction of a charge transfer mechanism. This technique changes the absorption spectrum. However, the vibrational spectrum remains the same.
UV-Visible Diffuse Reflectance Spectrophotometry with Optic Fibers (FORS)
This study discusses the limitations and possibilities given by the UV-Visible Diffuse Reflectance Spectrophotometry with Optic Fibers (FORS) method for identifying gemstones.
FORS is an excellent alternative to these techniques. The advantage of the FORS technique is its employment in situ almost anywhere without steric constraints. It also works on opaque, translucent and transparent objects.
FORS is also a beneficial method for pleochroic gemstones since it is relatively easy to alter the measurement angle to verify different responses. The most important advantage of the FORS technique is that it provides an alternative to the classical methods in gemology, particularly where jewelry artworks cannot be moved, such as in museums.
Identification of Different Gemstones Using FORS
In this study, researchers identified various gemstones using FORS technology rapidly and easily compared to other techniques. Some of these gems are mentioned below.
Identifying Blue Gemstones
Sapphire is the most precious blue gemstone. It is blue due to intervalence charge transfer (IVCT) between Ti4+ and Fe2+.
Using the FORS spectrum, researchers characterized sapphire via identifying absorption bands at ca.706, 456, and 390 nm due to Fe3+ and a band at 570 nm due to Fe2+-Ti4+ IVCT. Moreover, two luminescent bands at 694 and 693 nm appeared due to their additional spectral features due to Cr3+ ions, appearing as a solitary sharp negative band in the FORS spectrum.
Another valuable blue stone is lapis lazuli. Its blue color is caused by the lazurite phase Na6Ca2(Al6Si6O24)(SO4,S,S2,S3,Cl,OH)2. IVCT mechanism of absorption between S3-. S2-, and HS3- radicals cause the FORS spectrum to be dominated by 600 and 400 nm bands.
Similarly, FORS was used to identify a variety of green gemstones, including emerald, alexandrite, chrome-chalcedony or mtorolite, malachite, and various pink gemstones. including rhodochrosite and rhodonite. Red and purple gemstones were also identified. FORS was able to differentiate between spectral features of ruby and ruby glass.
The study discusses several advantages and some limitations of the FORS method. It can identify a large number of gemstones. However, for some gemstones with several varieties (quartz, tourmaline, topaz etc.), a proper database can counteract the low diagnostic power of the method.
The main advantage differentiating this technique from the rest is its rapidness and ease of use, allowing a safe way of analyzing jewelry artwork without moving them from their locations.
Full databases with the broadest range of variations must be built for this approach to be used effectively. Building a spectrum database entirely compatible with the instrumental setup is the greatest option in addition to accessing databases that are accessible in the books and on the web.
Maurizio Acet, Elisa Calà, Federica Gulino, Francesca Gullo, Maria Labate, Angelo Agostino, and Marcello Picollo (2022) The Use of UV-Visible Diffuse Reflectance Spectrophotometry for a Fast, Preliminary Authentication of Gemstones. Molecules. https://www.mdpi.com/1420-3049/27/15/4716