Introduction

Gemology is a complex field that involves multiple disciplines such 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 processed. Due to the fact that the objects to be analyzed are usually macroscopic gemstones with great economic value, and sometimes may be embedded in jewelry or packaged in containers, they must be completely and completely intactDistinguish from non-invasive methods. Raman spectroscopy technology is a completely non-invasive, non-contact, and sample free technique based on the Raman scattering phenomenon. It analyzes the molecular structure and chemical composition by measuring the small frequency changes of incident light caused by molecular vibrations in the sample. Raman spectroscopy technology also has advantages such as short measurement time and low sample demand. Therefore, it is highly favored in the field of gemology. In the past, there were very fewThe laboratory can apply Raman spectroscopy to gemstone research, but in the past decade, with the development ofAOPU Tiancheng ATR3000The emergence of portable Raman spectrometers represented by has fully demonstrated the application advantages of Raman spectroscopy technology in this field. In recent years, many gemological laboratories have been equipped with Raman spectrometers to study emeralds, diamonds, gemstone inclusions, and inlaid gemstones, especially using portable Raman spectrometers to analyze immovable objects such as gemstones on cultural relics. This article will explore the application of portable Raman spectroscopy in gemstone research, with a focus on its significant achievements in gemstone classification, identification, and field analysis. I believe that the introduction of this advanced technology can provide a new perspective for research in the field of gemology, injecting new vitality into the scientific research and identification of gemstones.

1、 Application of Raman Spectroscopy in Gemology: Identification

When we see a gemstone, the * * thing we want to know is very simple, which is: What is this gemstone in front of us? Many gemstones belong to specific mineral types, such as diamond, which has a chemical composition of carbon; Beryl, yellow jade, and zircon belong to silicate minerals, while ruby and sapphire are mainly composed of metal oxides. In this case, a portable Raman spectrometer can be used for rapid testing of gemstones, making it easy to identify the mineral types of the gemstones. After obtaining the spectrum, it can be compared with a standard spectral library to obtain information about the analyte. The following figure shows the Raman spectra of aquamarine, sapphire, tanzanite, forsterite, and cordierite. By the number and position of Raman peaks, different gemstones can be visually distinguished.

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

2、 Application of Raman Spectroscopy in Gemology: Composition

In many cases, gemstones belong to a series of mineral groups composed of heterogeneous materials, with their chemical composition continuously changing within a specific range. The extreme and end terms of the composition range are called end elements. Generally speaking, gemstones in mineral groups can be regarded as solid solutions of end elements. Tourmaline and garnet are examples of gemstones that do not correspond to mineral types but to mineral groups. Understanding the actual composition of a gemstone (i.e. the percentage of each end member in the mineral group) is crucial for accurately assessing its value and origin.

Here is an example of garnet, a gemstone belonging to the garnet mineral group can usually be written as: A3B2(SiO4)3Among them, A represents Ca2+、Fe2+、Mn2+、Mg2+， B represents Al3+、Cr3+、Fe3+The end members in the garnet mineral group are roughly divided into pyrophyllite (Pyr, Mg)3Al2Si3O12）Aluminum garnet (Alm, Fe)3Al2Si3O12）Manganese aluminum garnet (Spe, Mn)3Al2Si3O12）Representing aluminum garnet and calcium chromium garnet (Uva, Ca)3Cr2Si3O12）Calcium iron garnet (And, Ca3Fe2Si3O12）And calcium aluminum garnet (Gro, Ca)3Al2Si3O12）Represented by calcium garnet, the gemstones corresponding to pure end elements are rare in nature, so common garnets are solid solutions composed of these end elements. The Raman spectra of garnet and garnet series are shown in Figure 2. By modeling the data using Matlab, the composition ratios of different end members of garnet species can be analyzed, providing a basis for their value evaluation.

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

3、 Application of Raman Spectroscopy in Gemology: Imitation, Synthesis of Gemstones, and Artificial Processing

As mentioned earlier, one of the most important challenges in gemology is the enhanced processing methods for identifying and recognizing natural gemstones, such as heating, crack filling, radiation exposure, coating, bleaching, dyeing, and so on. For example, some broken rubies were filled with high refractive index glass to improve their clarity, and Fan et al. found lead glass at 1500cm in the Raman spectrum of rubies-1The wide characteristic peaks (Figure 3) provide a powerful tool for manually processing gemstone recognition.

Figure 3 Raman spectra of natural ruby and filled glass: natural is the intrinsic peak of ruby, and P1 and P2 are the characteristic peaks of fillers on ruby.

On the other hand, for Raman spectroscopy, detecting replicas is a relatively simple task because compared to real gemstones, their composition is often different. For example, cubic zirconia, which is often used to impersonate diamonds and other gemstones, has a completely different chemical composition from diamonds and will present a different spectrum. Figure 4 shows the Raman spectra of diamond, its most common replica cubic zirconia, and another replica strontium titanate.

Figure 4 shows the Raman spectra of diamond and its imitations, cubic zirconia, and strontium titanate, arranged from top to bottom as diamond, cubic zirconia, and strontium titanate.

summary

Raman spectroscopy means speed, sensitivity, and non-destructive testing, meeting the requirements of gemology. By using a Raman spectrometer equipped with different lasers in the visible and near-infrared wavelength ranges, information about the properties, composition, structure, and purity of gemstones can be obtained to identify inclusions and detect enhanced treatments. Recent studies have shown that Raman spectroscopy can also be used to obtain quantitative information on the composition of gemstones. 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).ATR3000 portable spectrometerThe research scope can also be expanded to analyze gemstones preserved in museums or gemstones embedded in archaeological or historical artworks.

Raman spectroscopy can be used as a standard tool in gemological laboratories, and gemstone sellers should widely use Raman spectroscopy for rapid identification. Raman measurement is both fast and cost-effective, demonstrating its application in the field of gemological research and making important contributions to the study of gemstone genesis and origin. The analysis of slight changes in vibrational spectra and precise study of microfluidics and solid inclusions can reveal important details of mineral geological history.

In the long run, 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 popularization of mineral Raman spectroscopy comprehensive databases, automation of routine measurements of a large number of samples, and the possibility of user-friendly software and hardware systems to quickly identify gemstones and their components with sufficient confidence using user methods will contribute to the spread of this technology outside of research laboratories.

References:

[1] Bersani, D. and Lottici, P.P., 2010. Applications of Raman spectroscopy to gemology. Analytical and bioanalytical chemistry, 397, pp.2631-2646.

[2] Fan, J.L., Guo, S.G. and Liu, X.L., 2009. The Application of confocal micro-Raman spectrometer to nondestructive identification of filled gemstones. Spectroscopy Letters, 42(3), pp.129-135.

[3] Bersani, D., Andò, S., Vignola, P., Marino, I.G. and Lottici, P.P., 2009, August. Micro‐Raman determination of the composition of ugrandite garnets. In AIP Conference Proceedings (Vol. 1163, No. 1, pp. 35-43). American Institute of Physics.

[4] Aponick, A., Marchozzi, E., Johnston, C.R. and Wigal, C.T., 1998. Determining the authenticity of gemstones using Raman spectroscopy. Journal of chemical education, 75(4), p.465.