Application of Raman Spectroscopy in Gemological Research

Introduction

Gemology is a complex field involving disciplines as diverse as natural sciences, art, history and archaeology. One of the most important issues in gemological research is the identification of gemstones, including their type, purity, origin and whether they have been artificially treated. Since the objects to be analyzed are usually macroscopic gemstones, which have great economic value and may sometimes be mounted on jewelry or enclosed in containers, identification must be carried out in a completely non-destructive and non-invasive manner.

Raman spectroscopy is a completely non-invasive, non-contact technology that does not require any sample preparation. It is an analytical method based on the Raman scattering phenomenon and analyzes the molecular structure by measuring the tiny frequency changes of incident light caused by molecular vibrations in the sample. and chemical composition. Raman spectroscopy technology also has the advantages of short measurement times and low sample requirements. Therefore, it is very popular in the field of gemology.

In the past, few laboratories could apply Raman spectroscopy to gemstone research. However, in the past decade, with the advent of portable Raman spectrometers represented by Optosky ATR3000, Raman spectroscopy technology has fully demonstrated its application in this field. application advantages in the field. In recent years, many gemological laboratories have been equipped with Raman spectrometers to conduct research on emeralds, diamonds, gem inclusions, and inlaid gemstones. In particular, portable Raman spectrometers are used to analyze immovable objects such as gemstones on cultural relics. . This article will discuss the application of portable Raman spectrometers in gem research, focusing on its remarkable results in the classification, identification and field analysis of gems. It is believed that the introduction of this advanced technology can provide a new perspective for research in the field of gemology and inject new vitality into the scientific research and identification of gemstones.

Applications of Raman spectroscopy in gemology:

Identification

When we see a gemstone, the first thing we want to know is simple: what is this gemstone in front of us? Many gemstones belong to clear mineral categories. For example, the chemical composition of diamonds is carbon; beryl, topaz, and Zircon is a silicate mineral, while ruby ​​and sapphire are mainly composed of metal oxides. In this case, a portable Raman spectrometer can be used to quickly test the gem. The mineral type of the gem can be easily identified. After obtaining the spectrum, compare it with the standard spectrum library to obtain information about the object to be tested. The picture below shows the Raman spectra of aquamarine, sapphire, tanzanite, forsterite and cordierite. Through the number and position of the Raman spectrum peaks, different gemstones can be distinguished intuitively.

Figure 1 Raman spectra of some common gemstones (aquamarine and sapphire) and less common gemstones (tanzanite, forsterite and cordierite) obtained using 633 nm excitation

Element

In many cases, gemstones belong to a series of allomorphic mineral groups whose chemical composition varies continuously within a specific range. The extreme terms of the composition range are the so-called end members. Generally speaking, gemstones in the mineral group can be regarded as solid solutions of the end members. Tourmaline and garnet do not correspond to mineral types but to mineral groups. Examples of gems. Knowing the actual composition of a gemstone (that is, the percentage of individual endmembers in that mineral group) is important to correctly assess its value and provenance.

Here is an example of garnet. Gemstones belonging to the garnet mineral group can usually be written as: A3B2(SiO4)3, where A represents Ca2+, Fe2+, Mn2+, Mg2+, and B represents Al3+, Cr3+, Fe3+. The end members in the garnet mineral group are roughly divided into almandine garnet represented by pyrope (Pyr, Mg3Al2Si3O12), almandine garnet (Alm, Fe3Al2Si3O12) and spessartine garnet (Spe, Mn3Al2Si3O12), and almandine garnet represented by gyroscope. Grossular garnet is represented by grossularite (Uva, Ca3Cr2Si3O12), grossularite (And, Ca3Al2Si3O12) and grossularite (Gro, Ca3Al2Si3O12). Gems corresponding to pure endmembers are rare in nature, so common pomegranates Stones are solid solutions composed of these end members. The Raman spectra of rhodolite and yellow garnet series are shown in Figure 2. Data modeling through Matlab can analyze the composition ratio of different end members of garnet species and provide a basis for its value assessment.

Figure 2 Raman spectra of rhodolite (a) and yellow garnet (b) series garnets

Imitations, synthetic gemstones and artificial treatments

As mentioned previously, one of the most important challenges in gemology is the identification and recognition of enhanced treatments of natural gemstones, such as heating, fracture filling, radiation exposure, coatings, bleaching, dyeing, and more. For example, some cracked rubies are filled with high refractive index glass to improve their clarity. Fan et al. found a wide characteristic peak of lead glass at 1500cm-1 in the Raman spectrum of rubies (Figure 3), which is a sign of artificially processed gems. provides a powerful tool for identification.

Figure 3 Raman spectra of natural ruby and filled glass: natural is the intrinsic peak of ruby,

P1 and P2 are the characteristic peaks of the filler on ruby.

With Raman spectroscopy, on the other hand, detecting imitations is a relatively simple task because their compositions are often different compared to real gemstones, such as cubic zirconia, which is often used to pass off diamonds and other gemstones, and its chemistry The composition is completely different from that of diamond, so it will show a different spectrum. Figure 4 shows the Raman spectra of diamond, its most common imitation, cubic zirconia, and another imitation, strontium titanate.

Figure 4 Raman spectra of diamonds and their imitations, cubic zirconia and strontium titanate. From top to bottom, they are diamond, cubic zirconia and strontium titanate.

Summarize

Raman spectroscopy means speed, sensitivity and non-destructive testing, meeting gemological requirements. Using a Raman spectrometer, equipped with different lasers in the visible and near-infrared range, it is possible to obtain information on the nature, composition, structure and purity of gemstones to identify inclusions and detect strengthening treatments. Recent research has shown that Raman spectroscopy can also be used to obtain quantitative information on gem composition. Raman analysis is not limited to inorganic crystals, but can also obtain information on organic gemstone materials (coral, amber and pearls) or amorphous or nanocrystalline materials (obsidian, opal). The ATR3000 portable spectrometer can also expand the scope of research, allowing the analysis of gemstones preserved in museums or mounted on archaeological or historical artworks.

Raman spectroscopy can be used as a standard tool in gemological laboratories and should be widely used by gem sellers for rapid identification. Raman measurement is fast and does not require the purchase of instruments. It has shown its application in the field of gemological research and made important contributions to the study of the origin and provenance of gemstones. The analysis of slight changes in vibrational spectra and the precise study of microscopic fluid and solid inclusions can reveal important details about the geological history of minerals.

In the long term, the use of Raman spectroscopy with different techniques such as scanning electron microscopy, energy-dispersive X-ray spectroscopy or FT-IR spectroscopy will further expand the application of Raman spectroscopy in gemological research. On the other hand, the popularity of comprehensive databases of mineral Raman spectra, the automation of routine measurements of large numbers of samples, and the possibility to quickly identify gemstones and their compositions with sufficient confidence using user-friendly software and hardware systems will contribute to this Diffusion of technology beyond the research laboratory.