Written by Melissa Allen, article featured in AGTA PRISM Vol. 2, 2019
Traditional gemological testing of gem and treatment identification for the average gemologist has a limited number of tools such as the microscope, refractometer, polariscope, dichroscope, among others. They are especially limiting when we need to test gems mounted in jewelry and objects, and we are unable to access them fully. These tools have been our right hand and many times they give us what we need, but often they do not. We are then required to send what we cannot detect, know or understand to better-equipped laboratories which offer advanced testing. Until recently this capability was solely reserved for these laboratories mainly due to the price of necessary equipment. The progress in technology has allowed for the production of advanced testing equipment at a lower cost and smaller in size, so these machines are now making their way into small independent laboratories, stores, and appraisal offices. For the first time, tools such as Raman and FTIR spectrometers give gemologists the ability to learn, understand and test gem materials in an entirely new way, by bringing a new in-depth foundation of science to each of us.
Infrared and Raman spectroscopy techniques have the ability to accurately reach conclusions from identification to treatments that have otherwise been time-consuming or impossible to achieve with traditional gemological tools, especially when they are paired and used in conjunction with each other. The two methods are complementary to each other and provide fundamental characteristic vibrations that are used to determine the identification of the molecular structure (Larkin, n.d.). Therefore, they are both commonly known as “vibrational” spectroscopy. Raman gives its most precise results on materials with a crystalline structure, and FTIR is more proficient with amorphous and organic materials. Raman spectroscopy’s most valuable asset is that it offers accurate gem identification, it is non-destructive and able to be used on rough, polished and even mounted gems. It requires no sample preparation and provides quick results, usually within 20 seconds to 1 minute. It is likely the only gemological tool which can deliver a conclusive identification without the need for further testing. FTIR has many gemological applications, among them, it can identify diamond types and some of its treatments, detect fillers in gemstones and differentiate between certain natural and synthetic gems.
The Raman Effect
The Raman Effect is the shift in energy compared to a monochromatic radiation source, such as a laser, caused by inelastic scattering. When the monochromatic source interacts with matter, a scattering of photons will occur. Most of the scattered photons will have the same energy as the source (elastic scattering or Raleigh scattering). An incredibly small number of photons emitted (1 in 10 million) will have a different vibrational energy (inelastic scattering or Raman shift) if energy has been gained or lost by the molecules. Being that inelastic scattering is particularly weak, it is necessary to use an extremely powerful source to increase the number of photons that cause it. The Raman Spectrometer provides a very strong source that enables its detector to see and record this small portion of the scattered radiation. Each molecule has its own distinctive and unique vibrational energy which is determined by its chemical composition, molecular mass, and bonding factors. A Raman spectrometer can record this energy, known as “fingerprint”, and comparing against a database of known samples it allows us to identify practically every gem material.
Over the past 20 years, Raman spectroscopy has become more readily available due to the development of the laser, which has provided us with a compact and affordable, strong monochromatic light source that is needed to achieve the inelastic scattering. Prior to the laser, the only other option available was the monochromator, which is sizable and expensive to produce (Scarani and Åström, n.d.).
Figure 1 – Schematic of a modern Raman spectrometer setup.
- The laser emits a narrow monochromatic energy beam aimed at the sample.
- The sample material interacts with the laser beam and emits elastic and inelastic scattered
- A filter is then used to remove the elastic scattering, and only the inelastic scattering (the Raman “fingerprint”) will reach the detector of the spectrometer (Figure 1).
- A computer equipped with specific software will process the data and display the Raman spectrum.
Advantages of Raman in Gemology
In gemology, the use of standard testing methods and instrumentation often requires multiple confirmations for identification; the ability to use a single tool offers an enormous advantage. Raman spectroscopy is likely the most effective technique for identification of rough, polished, loose or mounted gems. Being that the Raman fingerprint is exceptionally accurate and precise as a means of identification, it is not necessary to conduct further analysis in order to reach a reliable and conclusive result. As metal does not produce Raman signal, the ability to quickly and accurately conclude the identification of mounted goods is a vast and unprecedented advantage. The strong reliability, combined to the non-invasiveness, lack of sample preparation and speed in obtaining the conclusive results are all factors that have pushed this technique to quickly become a standard in the protocols of almost all the gemological laboratories.
Besides the identification of gems, it is even possible to detect some treatments, infiltration with foreign substances is one of the most important. The practice of enhancing clarity and the color of gems by impregnation is extremely old and considered common for emeralds and jade but, especially in recent years, many other materials on the market have been found to have been treated in this way, tourmaline and garnet to name a few of them. There are varying types of fillers used, from the conventional cedarwood oil to the most common polymers like Opticon or Permasafe. As we know, every material has its own Raman spectrum or “fingerprint”, and fillers are not an exception. In the Raman scan of an impregnated gem, the spectra of both the gem and filler will show up together. If we have a reference database to compare our scan with, it will be easy to identify which peaks belong to the spectrum of the gem and which are coming from the foreign substance. Fissure fillers have two characteristic Raman spectroscopic areas, which do not interfere, with the emerald fingerprint peaks: 1200-1700 cm-1and 2800-3100 cm-1 (Kiefert, Hänni, and Chalain 2000).
Identification of the inclusions in gems has always been problematic to determine. Traditionally it was only possible to do so by using microscopic observation. With advancing technology in more recent years, a Raman spectrometer paired with a microscope gives us the ability to identify gemstone inclusions using a common focal point through the optical path. The technique is called “confocal micro-Raman spectroscopy”, and it is particularly effective in analyzing the inclusions in gems, providing valuable information on their origin, authenticity, conditions of formation, and treatments (Dao and Delaigue 2000).
Raman and Photoluminescence (PL)
For decades Raman analysis has been used and relied upon for gemstone identification, and it frequently demonstrates its usefulness in the gathering of photoluminescence (PL) spectra (Eaton-Magana and Breeding 2016). When a Raman spectrometer employs a visible monochromatic light source, it can produce PL reaction in gem material allowing the instrument to work as a PL spectrometer. Raman and PL spectra are collected simultaneously with the same instrumentation, and Raman peaks may possibly appear in PL spectra if the emission peaks are not too pronounced (Eaton-Magana and Breeding 2016).
Figure 2 – CVD synthetic diamond Raman and PL spectra
Among the most useful gemological applications, the PL technique grants us the ability to detect CVD synthetic diamonds, to see indications of HPHT treatment in type IIa diamonds, differentiate between natural, synthetic and heated spinel and between natural and synthetic emeralds. One of the most distinctive features of colorless CVD synthetic diamonds is the contamination of silicon occurring during the growth process. The negatively charged silicon-vacancy center or the SiV– defect (an atom of silicon bonded to a vacancy) is actually a doublet, centered at 736.6 and 736.9 nm and is very seldom seen in natural diamonds to the point that is largely considered a diagnostic characteristic of the CVD synthetic growth process (Figure 2) (Eaton-Magana and Breeding 2016).
In the PL spectrum of emeralds, a pair of emission peaks are always present due to chromium content and proven to be useful in the determination of the schist, non-schist, and synthetic origin of the gem.
Figure 3 – Photoluminescence (PL) spectra of natural and synthetic emeralds.
By evaluating the so-called R Line, which is located in a narrow range (680–685 nm) and by identifying its precise position, we can establish whether the stone is natural, coming either from schist or non-schist deposit, or its synthetic (Figure 3) (Thompson et al. 2014).
The PL spectrum of spinel is virtually always characterized by a distinct emission peak and some related bands due to chromium. In natural unheated material the main chromium PL peak is narrow and centered at 685.5 nm while in heat treated spinel the such is broader and its ZPL, Zero Photon Line, is shifted towards higher wavelengths (Zagorevskii 1999).
Disadvantages of Raman Spectroscopy
- It is problematical to obtain an accurate Raman spectrum on amorphous materials due to the lack of their crystalline structure.
- Most natural and synthetic gems have the same “fingerprint”, so it is impossible to differentiate them with Raman spectroscopy, except for spinel and few others due to the slightly different chemical composition.
- Black materials can be difficult or impossible to identify because they tend to absorb their own Raman scattering. In most cases, the signal is so weak it is confused with the background noise of the spectrum.
Fourier Transform Infrared Spectroscopy
Fourier Transform Infrared Spectroscopy (FTIR) is a vibrational technique that measures the absorbance, transmittance, and reflectance of infrared radiation resulting from its interaction with the gem. The Fourier-Transform technique has many advantages over traditional infrared spectroscopy due to the use of the Michelson interferometer, such as its higher power output and the capability of quickly scanning all the frequencies of the infrared source at the same time (Åström and Scarani, n.d.). The sampling setup and operating concept is quite simple: The gem is placed on the sample stage, between the infrared source and the detector. The detector receives the energy that is not absorbed by the sample, and this presents us with the absorbance spectrum displayed on a monitor. It sounds simple and quite easy to get a result, but we need to keep in mind that we are testing gems and they are cut and faceted with the purpose of interacting and reflecting with light. Most often they are not simple flat objects, and the cut can significantly affect the path of the beam, and at times it can be impossible to align the stone in such a position to let even a small portion of the radiation pass causing the beam to become diverted and unusable. For this reason, in gemological applications, the standard transmission setup is much less suitable, and different sampling methods have been proven to be more effective. The most commonly
used methods are diffuse reflectance (DRIFT) and specular reflectance (Figure 4).
DRIFT sampling technique is the most frequently used technique by FTIR in gemology today. It was discovered that by using a module with a gold-covered stage, coupled to a custom designed ellipsoid mirror helped to avoid the issues caused from the beam deviations produced by the facets of the gem. The beam passes through the gem twice before it reaches the detector. This causes the beam to become significantly diffused and aids in the increase of the signal intensity (Åström and Scarani, n.d.).
The specular reflectance technique is utilized for material identification as well as detection and recognition of foreign substances used to fill gem materials. The infrared beam is reflected by the surface of the gem, giving us a reflectance spectrum, which is similar to that of the Raman “fingerprint”. The FTIR spectrum features its “fingerprint” area usually between 400 and 1450 cm-1. This technique is predominantly essential to compensate for the inadequacy of Raman in the identification of black and amorphous materials, for example, black spinel. An important thing to note is that diamond does not feature a specular reflection spectrum (Åström and Scarani, n.d.).
Advantages of FTIR in Gemology
FTIR spectroscopy has multiple essential functions in gemology. Firstly, FTIR is the traditional and well-established method used to classify diamonds (Breeding and Shigley 2009). The area between 1332 and 400 cm-1, provides us with a characteristic spectrum due to the presence of nitrogen. Diamonds with sufficient nitrogen detectable by IR spectroscopy are classified as type I, whereas the ones lacking enough nitrogen to be detectable by IR spectroscopy are classified as type II (Breeding and Shigley 2009). FTIR has allowed for the analysis of “a” (aggregated) and “b” (isolated) nitrogen in type I diamond. It is, therefore, possible, through this technique, not only to know exactly the type of diamond but also, by using specific algorithms, to calculate the percentages of aggregates A and B and of single nitrogen that is present. The absence of absorption peaks, due to the lack of nitrogen and boron, in this specific area indicates the diamond type as IIa. When Boron impurities are detected in diamonds, they are characterized as type IIb (Åström and Scarani, n.d.). FTIR also aids in the detection and identification of diamond treatments such as irradiation.
One of the essential gemological application of FTIR is the detection of fillers used for clarity enhancement such as oils and polymers in emeralds, jade, and other gemstones. FTIR spectroscopy has been the most common technique to identify various filler substances from organic oils to polymer resins. (Hainschwang 2002).
One of the most common tasks we face in gemology is the identification of heating in corundum. Although traditionally we have been able to discover clues of possible heat treatment by microscopic observation of inclusions and their morphology, FTIR can provide significant indicators and may help the gemologist especially when microscopic observations are inconclusive. There are a number of foreign minerals (often not visible with the microscope) that feature specific absorption bands in the FTIR spectrum. Kaolinite, diaspore, gibbsite, calcite, boehmite, goethite are the most important ones. When their presence is highlighted in the FTIR spectrum, we can conclude that the stone has not been heated because they would have been destroyed otherwise (Hughes and Hughes 2017). A few more essential indicators of heating in corundum are the structurally bonded OH series, one is the 3309 cm-1, and another one is the 3160 cm-1. One classic example is the pronounced presence of the 3309 series in a metamorphic sapphire lacking any other band from contaminants as described above (Figure5). In this case that is a good indicator that the stone has been heat treated in reducing atmosphere. The presence of even a minor feature at 3232 cm-1 is diagnostic of heating in rubies while a well-structured series at 3160 cm-1 in a Winza ruby or a Kashmir blue sapphire or a yellow sapphire is a useful sign showing the absence of treatment (Hughes and Hughes 2017).
Synthetic emeralds can be easily be differentiated from natural ones by using their FTIR absorption spectra. The features of various types of synthetics are so precisely identified that in many cases it is possible to distinguish between the manufacturers (Figure 6) (Thompson et al. 2014).
As Raman, FTIR is nondestructive and does not need any sample preparation. FTIR is currently the only technique having the capability of identifying synthetic amethyst. When conventional gemological methods are inadequate, FTIR spectroscopy at a high resolution (0.5 cm–1) can accurately separate the material currently on the market, including some rare and unusual synthetics. (Karampelas et al., 2011).
Synthetic flux Alexandrite can be differentiated from its natural counterpart. The principal differences can be observed in the range of 2000-4200 cm-1 (Stockton, 1988).
Amber and copal are challenging to distinguish between with standard gemological instruments. FTIR spectra of both materials present differences which are in many cases diagnostic. Most of the times it is even possible to identify Baltic amber by the typical feature named as “Baltic Shoulder” (Beck et al. 1965) in the 1259-1184 cm-1 Range.
Disadvantages of FTIR Spectroscopy
- The sampling chamber of an FTIR can present some limitations due to its relatively small size.
- Mounted pieces can obstruct the IR beam. Usually, only small items as rings can be tested.
- Several materials completely absorb Infrared radiation; consequently, it may be impossible to get a reliable result.
In many cases, classic or traditional gemological tools are not able to solve the complex and numerous issues produced by the development of new treatments and synthetics. For this reason, a more analytical approach is required, and the use of spectroscopy is currently essential for accurate and thorough gem and treatment identification. Raman and FTIR spectroscopy are two reliable and well-established techniques in modern gemological laboratories, and since their advantages and disadvantages are mostly complementary to each other, these two spectrometers are virtually always both present in laboratories. In some cases, they are not only necessary but indispensable. For example, the study of the defects in diamonds to identify treatments and synthetics (FTIR), the fast identification of practically any gem material, regardless if it is rough, measuring less than 1 mm or mounted (Raman). The quick screening of colorless/near to colorless synthetic CVD diamond, emerald or spinel (PL), the identification of inclusions in gemstones (Confocal Micro-Raman), by detecting contaminants that would otherwise be undetectable to aid in the determination of heat treatment in corundum (FTIR), and for many more critical gemological applications.
All the figures included in this paper are covered by copyright and published with the written permission of MAGILABS and their authors, Alberto Scarani and Mikko Åström.
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