FTIR spectroscopy

Fourier-transform infrared spectroscopy, FTIR for short, examines the absorption of infrared light. This light excites vibrations in molecules and crystal lattices. Which wavelengths are absorbed depends on the chemical bonds – the spectrum is a kind of fingerprint.

How the measurement is made

The instrument sends infrared light through the stone or measures reflection from its surface. An interferometer and a mathematical conversion, the Fourier transform, produce a spectrum over a broad range in a short time. The measurement is non-destructive.

What FTIR is used for

• Fillings: oils, resins and polymers in emeralds and other stones show their own absorption bands; it is often possible to tell whether oil or artificial resin was used
• Diamond type: nitrogen and boron determine whether a diamond is type Ia, Ib, IIa or IIb – an important basis for testing for synthesis and treatment
• Water and hydroxyl groups: in corundum, beryl or quartz they offer clues to formation and possible treatments
• Identification: for materials such as jade, turquoise or organic gems the spectrum helps with identification

The limits

The sample sets limits: very thick or dark stones absorb infrared almost completely, and mounted stones can only be measured to a limited extent. Small amounts of a filler may lie below the detection limit. As with all methods, the result is combined with other findings.

Importance for diamonds

For diamonds, FTIR measurement is now a standard step. Because most natural diamonds contain nitrogen, a nitrogen-free type IIa diamond stands out and is examined specifically for synthesis and HPHT treatment. FTIR alone does not decide the question, but it shows where to look more closely.

In brief

FTIR shows the vibrations of chemical bonds in the infrared. The method detects fillings, determines diamond type and supports identification. Thick, dark or mounted stones set limits to it.

At NOBILITH

When a laboratory report states the type and amount of filler in an emerald, this is often based on FTIR. The statement appears in the dossier and also determines the care guidance.

Sources

• GIA: Gems & Gemology, articles on FTIR and diamond types
• SSEF: explanatory notes on emerald treatment