Instruments for Assessing Gem Diamonds

DTC has developed three screening instruments for examining and evaluating diamonds rapidly:

• DiamondSure™
• DiamondView™
• DiamondPLus™

The DiamondSure

The DiamondSure measures the absorption spectrum of a faceted gem diamond. The majority of natural near-colourless diamonds are type Ia, and absorb strongly in the UV spectrum at wavelengths less than approximately 330 nm. Many also contain sufficient N3 centres to be detectable in a room-temperature absorption measurement. Near- colourless type IIa diamonds, on the other hand, have neither of these absorption systems present. In the case of fancy yellow diamonds, a natural cape diamond would exhibit strong N3 absorption whereas a synthetic yellow diamond has a quite different absorption spectrum. Using these, and similar criteria, the DiamondSure will specifically identify about 98% of natural diamonds and refer all synthetic diamonds for further tests. About 2% of natural diamonds will also be referred for further tests. The instrument will explicitly identify type IIa diamonds (natural, synthetic HPHT and synthetic CVD) as well as synthetic Moissanite (a diamond simulant).

Type IIa diamonds are very rare, and, in a series of tests on 550,000 polished natural diamonds, DTC found that 98% ‘passed’ and could confidently be identified as natural without any further measurements being performed.

The DiamondView

The DiamondView instrument generates a luminescence image from the surface of a diamond by illuminating the stone with UV with wavelengths less than 225 nm. An image is also generated 0.1 s after the UV source has been turned off to check whether the diamond displays long-lived phosphorescence.

The standard screening sequence is that diamonds that have not been passed by the DiamondSure are examined using the DiamondView. Natural type IIa diamonds exhibit a blue luminescence that originates from a network of dislocations. HPHT synthetic diamonds that contain some nitrogen usually display green, yellow and blue luminescence, in distinct geometric growth patterns, from the characteristic growth sectors. Type IIa HPHT synthetic diamonds have predominantly blue emission, and show strong phosphorescence that is very rarely seen in natural specimens.

The majority of CVD diamond, although classified as type IIa, generally contains small concentrations of single substitutional nitrogen. Vacancies are introduced as part of the CVD growth process, and so these specimens contain sufficient concentrations of (N–V) and (N–V)0 centres to produce a strong orange luminescence, frequently with characteristic striations. It is technically possible to produce high-purity CVD diamond which shows very weak blue luminescence; however, this does not originate from dislocation networks as in natural type IIa diamond, but from dislocation bundles which have a different appearance. CVD diamond of this quality is, in any case, unlikely to find its way into the gem market; the absence of nitrogen makes the growth rate very low, and the resulting diamond is prohibitively expensive for gem applications.

The DiamondView, then, confirms the identification of yellow HPHT synthetic diamonds; it also allows differentiation of the HPHT synthetic type IIa diamond, the CVD synthetic type IIa diamond and the natural type IIa diamond. There is, today, a very real possibility that some near-colourless diamonds, identified as natural type IIa, were origi- nally brown and have been colour-enhanced by HPHT annealing.

The DiamondPLus

The DiamondPLus is an instrument developed by DTC to identify colour-enhanced type IIa diamonds. It measures the photoluminescence (PL) spectrum of the diamond using excitation with more than one laser. The measurement has to be carried out with the diamond cooled using liquid nitrogen for the PL lines to be sharp enough to be identified.

In DTC tests, the DiamondPLus successfully identified all colour-enhanced type IIa diamonds but also 'referred' approximately 30% of natural type IIa diamonds that had not been treated. These referred diamonds require more sophisticated tests and careful visual inspection. The operation of the DiamondPLus and further measurements required have some outlined criteria and limitations that were discussed previously. The limitations explain why there is a chance that an untreated diamond may be wrongly categorized as colour-enhanced.

The DiamondPLus is specifically engineered by DTC to examine polished diamonds and provide results within approximately 30 seconds. Other instruments on the market for measuring Raman and PL spectra can yield the same information but take longer to obtain a result.

Note: The results obtained from the DiamondPLus and other similar instruments are subject to limitations and may not provide an accurate assessment of the diamond's authenticity. Careful visual inspection and more sophisticated tests may be required to confirm the diamond's category. 

Summary: Diamond Gem Trade

During the last 15–20 years, there has been increasing interest in the diamond gem trade in fancy coloured diamonds. However, with the development of commercially viable production techniques for HPHT synthetic diamonds and the introduction of the HPHT process for the colour enhancement of brown diamonds, it is essential to differentiate between natural, naturally coloured diamond, natural treated-colour or colour-enhanced diamonds and diamonds produced by HPHT synthesis or CVD to retain consumer confidence in the diamond trade.

The growth of gem-quality diamonds by CVD has also been demonstrated, and this process may become commercially viable in the future. To determine the diamond type and whether the colour is natural or artificial, a methodology for examining diamonds spectroscopically has been developed, and in most cases, it is possible to differentiate between the different types of diamonds.

"Trust and transparency are essential in the diamond industry."