Detecting Treated, Synthetic, Colour- Enhanced and CVD Diamond

 

Detecting Treated, Synthetic, Colour-

Enhanced and CVD Diamond

At the present time large numbers of natural diamonds are treated, using radiation, on its own, or followed by heating at approximately 800 °C to produce green, yellow and orange diamonds, and possibly a few pink stones. Small quantities of HPHT synthetic diamonds are appearing in the diamond trade; most of these are fancy yellow and some are irra- diated and annealed to generate fancy pink colours. Colourless and blue HPHT synthetic diamonds are more difficult to manufacture, and are seen in only very small numbers. Significant numbers of natural brown diamonds are being colour-enhanced to produce near-colourless stones, from type IIa starting material, and fancy yellow and fancy yellow/green colours, from type Ia starting material. Because fancy colours are rare, the latter would immediately arouse suspicion, but of far greater concern is that any near-colourless diamond has potentially been colour- enhanced. At present there is no commercial production of CVD gem- quality diamond, but it is technically possible to produce near-colourless and fancy blue stones by this process. Perhaps in the future such material will be encountered in the diamond trade. In this section the range of colours that may be encountered is summarized, and methods are outlined for deciding whether a diamond is natural or synthetic and whether a colour is natural or artificial. In some cases reaching such a decision is straightforward, and in others it is extremely difficult. 

Visual Observation

Important clues about the type of diamond, and whether the colour is natural can often be obtained by visual inspection with a 10 loupe. We consider two important examples in the sections below. Visual observation of the distribution of luminescence may also be valuable, and is considered later.

HPHT Synthetic Diamond

Impurities are incorporated in very different concentrations in the different growth sectors of HPHT synthetic diamond, and this is certainly the case with nitrogen and with boron which cause the yellow and blue colours, respectively. The uneven distribution of these colours is therefore a strong indicator that the diamond is not naturally coloured.

HPHT-Processed Natural Brown Diamonds

In the HPHT colour enhancement process, temperatures around 2300 °C may be used. Very few of the standard HPHT presses, designed for diamond synthesis at lower temperatures (c.1400 °C) are able to reach the necessary pressures where diamond is the stable phase of carbon at this temperature. Although the diamonds only experience these conditions for a few minutes, the areas of diamond around inclusions, cracks and feathers can exhibit severe graphitization. Observation of such regions of graphitization would reinforce the conclusion that the diamond had been subjected to HPHT processing. Surfaces of faceted stones are badly etched during processing, and any residual etching on the repolished stone would also suggest that it has been processed. However, it is possible that some facilities can heat diamonds to the required temperature and maintain the pressure in the diamond-stable region; the absence of graphitization does not therefore indicate that the colour is natural.

Red and Pink Diamonds

Red and pink diamonds can be produced by irradiation and annealing of type Ib diamond – either natural or synthetic. The red colour is an extreme version of the treated pink, and results from giving a larger dose of irradiation than would be used to produce a pink stone. The optical band responsible for the colour is that due to the (N–V) centre with a sharp line at 637 nm and a structured band to shorter wavelengths. This absorption band is never seen in natural diamonds in sufficient strength to cause a pink or red colour, and so these treated pink stones are readily identified. In synthetic diamond the pink colour is uneven, because of the different nitrogen concentrations in the different growth sectors, but this is not easy to see in a strongly coloured stone. The 637 nm absorption can also be produced by HPHT processing of a type Ia diamond at sufficiently high temperatures to generate single nitrogen, followed by irradiation and annealing at 800 °C. It may be possible to see signs of the HPHT treatment from graphitized cracks and inclusions.

 

Orange Diamonds and Yellow Diamonds

Yellow diamonds are created through various processes:

• Radiation and annealing of type IaA diamonds to approximately 800°C
• HPHT processing of brown type Ia diamonds (to rather lower temperatures than those required to create yellow/green specimens)
• HPHT processing of near-colourless or pale cape yellow type Ia diamonds

Orange diamonds are rare and are considered fancy colored diamonds. They are formed from impurities in the crystal lattice of the diamond, typically nitrogen. The more nitrogen present, the more orange the diamond will be.

 

Radiation and Annealing of Diamond

Radiation and annealing of type Ia diamonds produces the H3 and H4 absorption bands which are responsible for the yellow colour. If a heavy irradiation is used, the diamond becomes almost opaque at wavelengths less than approximately 500 nm, and may then have an orange or even reddish brown colour. The treatment also produces a sharp absorption line at approximately 595 nm. This line can be eliminated by annealing the diamond to approximately 1000 °C, but, as the 595 nm line anneals out, two further absorption lines in the near infrared spectrum are created, known as H1b (2024 nm) and H1c (1934 nm). It is exceedingly rare to find any of these three absorption lines, or the H4 line, occurring naturally in diamond, and it is sensible to regard as treated a stone in which the 595 nm, H1b or H1c lines are found. The H1b and H1c lines can be eliminated if the diamond is annealed further to approximately 1400 °C, but then the H2 absorption is produced, giving the diamond a green hue, and again providing evidence of treatment.

HPHT-Processing of Type Ia Brown Diamonds

The yellow diamonds produced by HPHT processing of brown type Ia diamonds generally still have some brown component remaining, and, in particular, the board band at approximately 560 nm may still be present. In addition, the stones exhibit the coloured graining characteristic of the starting material and may have graphitized inclusions. Intense absorp- tion in the H3 band is extremely rare in naturally coloured diamonds, and it is this feature that would first arouse suspicion.

 

HPHT Processing of Type Ia Pale Yellow Diamonds

The absorption spectrum produced by HPHT processing of near-colourless or pale cape yellow stones is virtually identical to that observed in type Ib diamonds, and, if the processing conditions are optimized, a colour similar to that of natural ‘canary yellow’ diamond can be obtained. Unless a processed diamond contains other clues, such as graphitized cracks or inclusions, it is therefore difficult at present to be sure that the colour has been created artificially.

Green Diamonds

Green diamonds are a unique type of diamond that can be produced through various methods. These methods include:

• Radiation damage without annealing
• Radiation damage of type Ia diamonds followed by prolonged annealing at 1400 °C
• HPHT processing of brown type Ia diamonds using very high temperatures

The colour of green diamonds is caused by different types of absorption depending on the method used to produce them:

• GR1 absorption for radiation damage without annealing
• Combination of H2 and H3 absorption for radiation damage of type Ia diamonds and HPHT processing of brown type Ia diamonds

 

Irradiated Diamonds

Very few diamonds owe their green colour to naturally occurring GR1 absorption. The Dresden Green is one, but the intensity of the band is extremely low; at the peak of the band near 625 nm the absorption coeffi- cient is no more than 0.2 cm 1 above the background. The diamond only appears to have a green tint by virtue of its size (40.7 ct). The shape of the absorption spectrum of the Dresden Green differs in no perceptible way from the absorption spectra of diamonds that have been artificially coloured using radiation. Consequently gem testing laboratories are reluctant to issue origin-of-colour certificates for green diamonds contain- ing the GR1 absorption. However, the author is of the opinion that the Dresden Green, or some other diamond known to have GR1 absorption through natural processes, should be used as a benchmark, and that any diamond which exhibited a large GR1 absorption coefficient at 625 nm, compared with the benchmark, should be regarded as a ‘treated green’.

 

HPHT-Processed Type Ia Brown Diamonds

To the author’s knowledge, H2 absorption does not occur in natural diamonds to a sufficient extent to produce a green colouration. Any diamond that is coloured green or yellow/green because of the presence of H2 absorption should therefore be treated as suspect. The natural brown type Ia diamonds that successfully respond to HPHT processing frequently have low concentrations of nitrogen. Consequently, after pro- cessing, a green diamond will exhibit pronounced green (H3) lumines- cence on excitation with long-wave ultraviolet (LWUV); this is another indication. Before processing, such diamonds often exhibit brown grain- ing, and this graining is evident in the green and yellow/green colouring of the end product, and in the distribution of the luminescence, provid- ing further confirmation of colour enhancement.

 

Blue Diamonds

Information about how blue diamonds are formed

Irradiation of diamonds with electrons results in a blue or blue/green colour because of absorption in the GR1 band. Blue diamonds can also be grown by HPHT synthesis and by CVD. Blue colour in natural diamond is rarely, if ever, associated with GR1 absorption. The intense blue colours produced by boron doping of HPHT synthetic and CVD diamond are also unlike the relatively weak blue colour of natural type IIb diamond. In HPHT diamond the boron is taken up in different concentrations in the different growth sectors, and the uneven blue colour is easily noticed. It is therefore relatively straightforward to differentiate a natu- rally coloured natural blue diamond. In practice very few blue HPHT gem-quality diamonds are produced and there is currently no commer- cial production of gem-quality CVD diamond.

Colourless and Near-Colourless Diamonds

Following the GE/LKI revelation, every colourless or near-colourless diamond is potentially a type IIa diamond that has been colour-enhanced by HPHT processing of brown starting material. The first step in investigating such a stone must be to determine whether it is type II material. A simple procedure is to check the UV transmission at 254 nm. Diamonds that contain nitrogen in predominantly the B aggregate form are also transparent at this wavelength, and a specimen that is transparent at 254 nm must subsequently be examined spectroscopically in the defect-induced one-phonon region to check whether it is type IIa or type IaB. If the diamond is type IIa, further analysis will be required to assess whether it has been colour-enhanced. These analyses will involve both absorption and luminescence measurements. It is also possible to grow near-colourless diamonds by HPHT synthesis. 

 

Absorption Spectroscopy

Some nominally type IIa diamonds contain small concentrations of nitrogen, often predominantly in the B-form, which can be detected by very careful infrared spectroscopy. It may also be possible in some stones to detect an absorption feature known as N9, at 236.0 nm. If such a diamond is subjected to the HPHT conditions used to remove the brown colouration, some of the aggregated nitrogen decomposes to form single nitrogen, and it may be possible to detect this spectroscopically.

The presence of single nitrogen gives a very faint yellow colour to the diamond, and therefore, to produce the best colour grades, groups producing colour-enhanced type IIa diamonds are selecting the starting material, using infrared spectroscopy, to have a very low nitrogen concentration. Absorption spectroscopy is therefore not likely to provide definitive results for such stones.

If you are interested in learning more about absorption spectroscopy of diamonds, please consult with a diamond specialist.

 

Luminescence Spectroscopy

Many type IIa diamonds contain defect centres involving nitrogen and vacancies, but at much too low a concentration to detect using absorption spectroscopy. The presence of these centres, in particular, N3 H4, H3, 575 nm [i.e. (N–V)0] and 637 nm [i.e. (N–V)], can, however, be demonstrated using laser-excited luminescence spectroscopy. Significant luminescence is only observed at longer wavelengths than the wavelength of the laser, and the relative intensities of features in the luminescence spectrum will depend on the wavelength used for excitation. The laser also produces a ‘Raman spectrum’ which comprises a first-order line shifted by 1332.5 cm 1 from the laser line and a second-order band with shifts from the laser line of between 1900 and 2665 cm 1.

A number of gem-testing laboratories have examined the luminescence spectra of brown type IIa diamonds before and after HPHT processing, and compared these with the spectra of natural untreated near-colourless type IIa diamonds. There are a number of indicators which, when taken with other evidence, can detect the majority of colour-enhanced specimens:

• In untreated diamonds the intensity of the 575 nm line is normally larger than the intensity of the 637 nm line, if these lines are present, whereas the converse is true in the colour-enhanced diamonds. The explanation is straightforward: the 575 nm transition comes from the (N–V)0 centre, and the 637 nm transition comes from the (N–V) centre. In the untreated diamonds there is a negligible concentration of single nitrogen (which acts as a donor) and so the neutral (N–V)0 centre dominates. During the HPHT processing, some of the aggregated nitrogen (which is present in very small concentration) dissociates, providing single nitrogen donors which convert some of the (N–V)0 to (N–V). Occasionally an untreated near-colourless type IIa diamond is encountered in which the intensity of the 637 nm line exceeds that of the 575 nm line, so this criterion must be used with caution.
• In colour-enhanced diamonds the width of the 637 nm peak is frequently greater than the width of that peak in untreated diamonds (if present). The reason is that the colour-enhanced diamonds, which were originally brown, are plastically deformed. The random strain which results from this deformation can broaden the luminescence line to a greater extent than occurs in a naturally near-colourless diamond.
• Many sharp luminescence lines observed in some untreated type IIa diamonds disappear following HPHT annealing, but no new lines are produced. A few of the lines which disappear on HPHT processing are quite well defined, and consequently a diamond in which they are observed could, with reasonable confidence, be certificated as a natural colour. However, it would be dangerous to assume that a diamond in which they are absent has been colour-enhanced.
• Depending on the time and temperature used in the HPHT processing, some colour-enhanced diamonds show no luminescence when excited with a 514 nm laser. None of the untreated colourless diamonds examined to date shows a complete absence of luminescence. The absence of luminescence may therefore be an indicator of HPHT processing.

Although none of these tests is definitive, taken on their own, they do allow an accurate decision to be made in the majority of cases when combined with other observations (for example, the presence of graphitized inclusions, graining, etc.).

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."