Infrared Spectroscopy: Decoding the Vibrational Signature of Gemstones

Infrared Spectroscopy: Decoding the Vibrational Signature of Gemstones

Alongside Raman spectroscopy, infrared spectroscopy forms the second pillar of analytical work in a modern gemmological laboratory. The technique uses absorption of radiation lying just beyond the visible red to reveal the internal chemistry of a gemstone, expose treatments that often remain invisible under the loupe or microscope, and separate natural specimens from their laboratory-grown equivalents. Since its widespread adoption by gemmological laboratories in the 1990s, infrared spectroscopy has become essential to any rigorous assessment of diamond, corundum, emerald, jade or turquoise.

Origin and Discovery

British astronomer Sir William Herschel first observed the existence of invisible radiation lying beyond the red part of the visible spectrum in 1800, while measuring the heat released by each colour produced by a prism. The thermometer placed below the red recorded the highest temperature even though no visible light reached it. That single observation marked the birth of infrared spectroscopy. The earliest commercial instruments appeared in the late 1950s and operated by dispersing each wavelength one at a time. The real revolution came with the Fourier transform, a mathematical method combined with a Michelson interferometer that allowed the full spectrum to be acquired in seconds rather than hours. GIA adopted Fourier Transform Infrared spectroscopy, known as FTIR, in 1986, and the major international gemmological laboratories followed within the decade.

The Physical Principle Explained

Every bond between two atoms in a mineral vibrates continuously, somewhat like a microscopic spring connecting two beads. The frequency of that vibration depends on the nature of the atoms involved, the strength of the bond and the surrounding crystalline environment. When an infrared beam passes through a gemstone, certain wavelengths match these natural vibration frequencies exactly. Those wavelengths are absorbed by the stone, while the rest are transmitted and reach the detector. The resulting spectrum is a graph showing absorption intensity as a function of wavenumber, expressed in cm⁻¹, where each peak corresponds to a specific molecular vibration.

The spectral region most useful to gemmology lies in the mid-infrared, between 400 and 4000 cm⁻¹, with additional features in the near-infrared up to roughly 13 000 cm⁻¹. Aluminium-oxygen bonds in corundum, carbon-carbon bonds in diamond, silicon-oxygen bonds in beryl, and hydroxyl groups (OH) trapped within the crystal lattice each produce distinctive signatures. A single bond may exhibit several modes of vibration, including stretching, bending, rocking and twisting, multiplying the number of characteristic peaks observed.

Diamond Analysis

Diamond is the mineral for which infrared spectroscopy provides the greatest volume of information. Any trace of nitrogen or boron, down to a few atoms per billion carbon atoms, produces identifiable absorption peaks. This sensitivity allows diamond to be classified into types Ia, Ib, IIa and IIb, a classification that rests entirely on the concentration and configuration of these impurities.

Type information is decisive when separating natural from laboratory-grown diamond. Nearly all colourless diamonds produced by HPHT or CVD processes belong to type IIa, while fewer than two percent of natural colourless diamonds share that type. FTIR therefore directs the gemmologist immediately toward the additional analyses required when a rare diamond is suspected of synthetic origin. HPHT treatment leaves further markers behind, including a broad band near 1480 cm⁻¹ and a noticeable reduction of the platelet peak at 1364 cm⁻¹. Irradiation followed by moderate heating produces the H1a (1450 cm⁻¹), H1b (4935 cm⁻¹) and H1c (5165 cm⁻¹) defects, abundant in artificially coloured diamonds and uncommon in naturally coloured stones.

Heat Treatment Detection in Corundum

Ruby and blue sapphire heated to high temperatures often display, in the 3000 to 3700 cm⁻¹ region, a series of bands linked to stretching vibrations of hydroxyl groups incorporated or redistributed during treatment. The so-called 3309 series, made of peaks at 3309, 3232 and 3185 cm⁻¹ along with secondary peaks at 3367 and 3295 cm⁻¹, remains the subject of active research. The Spring 2025 issue of Gems and Gemology presented an updated characterization of these features and their behaviour at different treatment temperatures.

A more recent variant known as pressure heat treatment, or PHT, is used to intensify the blue colour of sapphires. PHT produces a broad peak near 3045 cm⁻¹ that does not appear in untreated stones. Conversely, the presence of a peak at 3161 cm⁻¹ is a reliable indication that a sapphire has not been heated at high temperature, and is particularly useful for ruling out beryllium diffusion. The presence of thermally unstable mineral inclusions such as diaspore, gibbsite or boehmite, each with its own infrared signature between 2300 and 3800 cm⁻¹, similarly confirms an unheated history.

Emerald, Jade, Turquoise and Other Applications

Emerald shows three informative spectral windows, between 2400 and 3100 cm⁻¹, 3400 and 4000 cm⁻¹, and 5000 and 5500 cm⁻¹. The absence of water-related absorptions distinguishes flux-grown synthetic emerald from natural or hydrothermal stones. A series of peaks attributed to chlorine, between 2400 and 3100 cm⁻¹, reliably identifies most laboratory-grown hydrothermal emeralds. FTIR also detects the resins, oils and polymers used to fill surface-reaching fractures, a treatment that is extremely common on commercial emerald.

For jade, and in particular jadeite, infrared analysis is the reference method for identifying so-called type B jade, that is, jade which has been bleached with acid and impregnated with polymer resin. Carbon-hydrogen stretching bands near 2856, 2873, 2928 and 2958 cm⁻¹ instantly reveal polymer presence even when the impregnation remains invisible under the microscope.

Turquoise, opaque by nature, cannot be analysed by direct transmission of the infrared beam. Gemmologists collect a tiny amount of surface powder, mix it with potassium bromide and press a translucent disc which clearly displays the signature of impregnation resins. The more recent attenuated total reflection approach, known as ATR, avoids this preparation and offers a useful non-destructive alternative. Baltic amber, natural alexandrite versus its synthetic equivalents produced by Czochralski pulling or flux growth, natural quartz against its hydrothermal counterpart, and many additional materials also benefit from FTIR as a discriminating tool.

In the Laboratory and Beyond

At Laboratoire Gem Quebec, infrared spectroscopy is one of the analytical techniques available, deployed when the situation calls for it, alongside Raman spectroscopy, UV-visible-near-infrared spectroscopy, X-ray fluorescence and, in certain cases, gamma spectrometry. The choice of instruments depends on the type of stone, the report tier requested and the specific questions raised by the case at hand. This approach prevents the trap of premature conclusions drawn from a single instrument. Infrared spectroscopy complements Raman spectroscopy by working on absorption while Raman relies on inelastic scattering. The two techniques are sensitive to different vibrational modes, and the data they produce are genuinely complementary.

The usefulness of FTIR extends well beyond gemmology. Analytical chemistry, food and beverage quality control, polymer identification in recycling operations, pharmaceutical analysis, art and heritage conservation, and even planetary science all rely on the same fundamental technique. Within gemstone work, however, it remains one of the three or four indispensable tools without which no serious determination of identity, origin or treatment can be made with confidence.