Fourier-transform infrared spectroscopy — universally abbreviated FTIR — is an analytical technique that measures how a gemstone absorbs mid-infrared light across a wavelength range of approximately 2.5 to 25 micrometres (wavenumbers roughly 400–4000 cm⁻¹). Because different molecular bonds absorb infrared radiation at characteristic frequencies, the resulting absorption spectrum functions as a molecular fingerprint: unique to a given material's chemistry and atomic structure. In gemmological laboratories, FTIR is a standard, non-destructive first-line test used to identify gem species, classify diamond types, detect treatments including heat, irradiation, and fracture filling, and distinguish natural stones from their synthetic counterparts.

How It Works

A conventional dispersive infrared spectrometer scans wavelengths sequentially, which is slow and susceptible to noise. FTIR instruments instead expose the sample simultaneously to a broad band of infrared radiation modulated by an interferometer — typically a Michelson interferometer — and record the resulting interference pattern as an interferogram. A mathematical operation called the Fourier transform converts this raw signal into a conventional absorption spectrum plotted against wavenumber. The principal advantages are speed (a full spectrum in seconds), high signal-to-noise ratio, and excellent wavenumber accuracy. Most modern laboratory instruments operate in transmission mode, reflectance mode, or — for surface-sensitive work — attenuated total reflectance (ATR), allowing analysis of polished stones without any sample preparation or damage.

Diamond Type Classification

FTIR is arguably most consequential in diamond analysis, where it is the definitive tool for classifying diamonds into their structural types based on nitrogen content and aggregation state.

• Type Ia: Nitrogen present in aggregated form (A- and B-centres). The vast majority of natural diamonds fall here. FTIR quantifies the relative proportions of A- and B-aggregates, which reflect the stone's thermal history and can inform origin discussions.
• Type Ib: Nitrogen present as isolated substitutional atoms. Rare in nature; common in many HPHT-grown synthetic diamonds, making FTIR a primary screening tool for synthetic detection.
• Type IIa: Negligible detectable nitrogen. Includes many of the world's most celebrated large diamonds. FTIR identification of Type IIa status is significant because these stones are candidates for HPHT colour enhancement — a treatment that can convert brownish Type IIa diamonds to colourless or near-colourless.
• Type IIb: Boron present as the dominant impurity, conferring natural blue colour and semi-conductivity. The Hope Diamond is a documented Type IIb stone.

GIA's laboratory uses FTIR as a routine screening step for every diamond submitted, and the identification of Type IIa or Type Ib characteristics automatically triggers additional testing.

Treatment Detection

Beyond diamond typing, FTIR is indispensable for detecting a range of clarity and colour enhancements across multiple gem species.

Fracture filling in emeralds: Cedar oil, Canada balsam, Opticon, and various synthetic resins each produce characteristic infrared absorption bands, particularly in the C–H stretching region near 2800–3000 cm⁻¹ and carbonyl bands around 1700–1750 cm⁻¹. FTIR can not only confirm the presence of a filler but often identify its chemical class, allowing laboratories such as Gübelin and SSEF to report the degree of filling with greater specificity than visual examination alone permits.

Polymer fillers in rubies and other corundum: Lead-glass filling — once widespread in low-grade rubies — and more recently polymer-based treatments produce distinctive infrared signatures absent in untreated stones. FTIR readily separates these from natural surface-reaching fractures.

HPHT treatment in diamonds: While HPHT processing itself does not leave a simple single-band signature, it modifies the nitrogen aggregation state in ways that FTIR quantifies. A Type IIa diamond showing no nitrogen absorption, combined with photoluminescence and UV-Vis data, builds the evidence base for an HPHT-treatment determination.

Irradiation and annealing: Certain irradiation-induced defect centres in diamonds produce characteristic absorption features detectable by FTIR, complementing photoluminescence spectroscopy in treatment disclosure.

Species Identification and Synthetic Detection

FTIR spectra are sufficiently species-specific to separate gem materials that may appear visually similar. Jadeite and nephrite, for instance, produce markedly different mid-infrared spectra reflecting their distinct silicate structures. Synthetic moissanite, occasionally encountered as a diamond simulant, is immediately distinguished from diamond by its characteristic silicon carbide absorption pattern. Synthetic emeralds grown by flux or hydrothermal methods show water and flux-inclusion signatures that differ from those of natural stones, contributing to origin and synthesis-method determinations.

Laboratory Practice

Major gemmological laboratories — including GIA, Gübelin Gem Lab, SSEF Swiss Gemmological Institute, and Lotus Gemology — incorporate FTIR as a standard instrument alongside UV-Vis-NIR spectrophotometry, photoluminescence spectroscopy, and energy-dispersive X-ray fluorescence. The non-destructive nature of the technique is essential: polished gems are placed directly in the beam path and returned to the client unaltered. Portable and handheld FTIR units have become increasingly available, though laboratory-grade bench instruments retain superior sensitivity and spectral resolution for definitive determinations. Interpretation of FTIR spectra requires trained analysts; automated spectral matching libraries assist but do not replace expert judgement, particularly in complex cases involving multiple treatments or unusual inclusions.

Further Reading

• GIA — FTIR Spectroscopy in Gem Testing
• GIA Gems & Gemology — Infrared Spectroscopy for Gem Identification
• Lotus Gemology — Laboratory Articles and Reports