Gemmological Institute of India has recently established the National Research Centre for Diamonds and Gemstones. The new centre is the proud owner of many research equipments among them is Fourier Transform Infrared Spectroscopy, (Nicolet 5700) along with Continuum Microscope having the Linkham heating and cooling stage. Although Infrared spectroscopes are known in gemmological field for more than 25 years, the object of this article is to acquaint the Indian Gem Trade with latest techniques in FTIR especially using microscopy having cryogenic techniques. The article explains what is infrared energy, Fourier Transform Infrared Spectroscopy and how it helps to identify synthetic diamonds, treated rubies and synthetic emeralds.

Fig. 1 Author placing the stone in the FTIR instrument

Fig. 2 Infrared region of the electromagnetic spectrum

Absorption features in the visible range are largely due to electron transitions, including those that generate colour, such as occur with chromium atoms in the corundum lattice and cause the colour of rubies. In the infrared, however, spectral features (absorption or transmission) generally arise from vibrations (as well as, in the far infrared, from rotations) of molecular and structural components of the crystals. For example, carbon in diamond and water when present in a gemstone has characteristic signals in the infrared.

rystal structures consist of atoms held together by chemical bonds. A possible analogy to describe these bonds is to think of them as springs connecting heavy weights such that the weights representing atoms have the ability to vibrate. Every group of atoms has a number of intrinsic vibration frequencies that correspond to rocking stretching, or bending of the bonds between the atoms of the group. In order to actually vibrate, the structure must extract energy from some source, in this case a beam of incident infrared radiation, giving rise to an absorption band.

In a typical dispersive instrument, the beam is split into two parts; one goes through the sample, while the other passes through a reference. Each beam is dispersed through a prism or a grating, and the absorption one particular wavelength is analyzed by partially obstructing the reference beam, until the same amount of energy goes through both beams. An FTIR spectrometer contains two parts that do not exist in classical dispersive instruments: a Michelson interferometer, which combines all the incoming infrared radiation into one “interferogram,” back into a spectrum. In the FTIR concept, the light is split into two halves by a semitransparent mirror (called a beam splitter). These two beams are then reflected back toward one another by two additional mirrors, one fixed, the other moving, so that the two beams “interfere” when they come back together at the beam splitter, giving rise to an interferogram.

Fig. 3 Diagram shows Interferometer

In the new Fourier transform instrument at Gemmological Institute of India, when the moving mirror is at exactly the same distance from the beam splitter as the fixed mirror (or the same distance plus an integer times half the wavelength), the interferences is constructive (i.e. the two intensities are added together). Otherwise, the interference is destructive. In this manner, a very good resolution is obtained without cutting down the amount of energy, a problem inherent to all older instruments. The data are digitized and processed using a Fourier Transforms program, which (through a sequence of many steps) basically transforms the final interferogram into a transmission spectrum and eventually into an absorption spectrum.

The FTIR spectrometer has a number of important advantages over the older dispersive instruments. Because the entire spectrum is recorded at the same time in the form of an interferogram, there is no need to mechanically scan one wavelength after the other. Thus, where 20 minutes were needed in the past to obtain a spectrum using a dispersive instrument, only a fraction of a second is required on an FTIR spectrometer. This allows the operator to run 100 or even 1000 spectra of the same sample in a very short time and then average the results in order to reduce the random “noise” and bring out weak bands that often contain essential information. There is also reduced heating of the sample, in contrast with dispersive instruments, and the consequent spectral perturbations are largely avoided.

In addition, the FTIR concept uses a laser, both to check the moving mirror displacements and as an internal reference of wavelength, another feature that is not found on the dispersive spectrometer.

further advantage is that the Nicolet 5700 spectrometer is monitored by a powerful computer that not only does the mathematics of the Fourier Transform, but also provides considerable flexibility to plot, display, store, and interpret spectra. Basically, then, an FTIR spectrometer is both faster and more accurate than a dispersive infrared spectroscope.

A transparent gemstone is usually cut in such a way that light returns to the eye, creating the brilliance and fire of the gem. The problem in spectroscopy is exactly the reverse. Light has to pass through the stone. This requires an adapter.

he most useful adapter is the microbeam chamber, where a curved mirror focuses the beam down to an area the size of a pinhead, or smaller. This intense focused beam can then be passed with relative ease through a very very tiny culet or the girdle of a stone to obtain a spectrum.

Gemstones have characteristic vibrational energies in the infrared that can be used for identification. However their spectral features are usually broader than for organic molecules. An analogy can be made with X-ray diffraction, where a pattern for a given mineral is the “fingerprint” of its atomic structure. For Infrared Spectroscopy, absorption associated with the vibrations of the crystal structure (“lattice vibrations”) is characteristic of the given combination of atoms constituting the gemstone.

Infrared spectroscopy reveals characteristic patterns for different types of diamonds (Ia, Ib, IIa, and IIb) in as much as both nitrogen and boron impurities trapped in the diamond lattice have absorption features in the mid-infrared. One of the most significant gemmological uses recently revealed for the near infrared is detection of the H1b and H1c bands (4941 and 5165 cm-1, respectively), which identify that a diamond has been irradiated and heat treated to produce or enhance yellow to brown coloration.

At the Gemmological Institute of India’s National Research Centre we are collecting data on various FTIR spectra of diamonds. We have found that our results are at par with all the major International Gem testing Laboratories. In the case of synthetic diamonds, we get 1344cm -1 typical absorption. Laboratory has had the privilege to test many synthetic yellow diamonds from “Gemesis” all of them show the typical 1344cm -1 as well as the synthetic pink diamond from Chatham which also absorbs the infrared radiation at 1344cm -1.

Fig. 6 Synthetic diamonds can be detected by FTIR

The new Synthetic Emeralds by hydrothermal power can be easily detected by FTIR. Synthetic Green Emeralds and Rubies show different spectra as compared to natural counterpart.

Water, either molecular (H2O) or as hydroxyl groups (OH) is combined in various forms in emeralds and many gemstones or is present as an impurity. These various forms of water have characteristic patterns in the mid-infrared and can be good indicators of structure, origin, or treatment. Preliminary results

Fig. 8 Synthetic emeralds can be identified by FTIR

Lead glass (Pb glass) treated rubies can be easily tested and differentiated from untreated ones. The extensive documentation of the characteristic infrared absorption spectra of glass treated and untreated is very helpful in recognizing impregnation in gemstones. One or more sharp bands will show up in the spectrum of an impregnated stone that are not present in the spectra of similar untreated stones. A detailed example of how this is applied to glass filled rubies appears below. Not only does infrared spectroscopy enable one to detect glass

Fig. 10 Lead glass (Pb glass) treated rubies can be identified by FTIR

At the Gemmological Institute of India, National Research Centre for Diamonds and Gemstones, the FTIR has become a very useful technique in detection of Synthetic Diamonds, Treated Rubies, Synthetic Emeralds and Synthetic Rubies. Research is continuously going on other gemstones including Turquoise, Jade and Opal for their precise identification. National Research Centre at Opera House, Mumbai is open to the gem traders as well as the public for the accurate identification and grading of diamonds and treatments in gemstones.