Diamonds

AnchorCert senior gemmologist Anu Manchanda discusses some of the treatments employed to improve the quality and appearance of diamonds.

High quality diamonds have a status all of their own, and the challenge of enhancing the apparent quality of a stone to increase its value has been tantalising scientists for centuries.

There are now a multitude of different ways in which the colour or clarity of a diamond can be irreversibly improved, often by a relatively inexpensive method. There is nothing wrong with this in principle, but the consumer would expect to pay less for a diamond which did not come by its good looks naturally. The respected international standards set by CIBJO, the World Jewellery Confederation, therefore require such enhancements to be disclosed. However, some of these improvements are very difficult to detect and there is scope for the unscrupulous if the industry does not remain alert. Anu Manchanda, senior gemmologist for AnchorCert Gem Lab,  has selected three relatively new treatments to focus on.

Diamonds arriving at AnchorCert may be loose or mounted. They are given a unique barcode number and photographed on receipt. Loose diamonds are weighed to three decimal places, and mounted diamonds are measured precisely using either a Presidium gauge, Schindler or C-Master gauge. An accepted formula will then be applied to estimate their weight to within a tolerance of five per cent of the true weight. For very small stones a Meleemeter gauge may be used. Once the weight has been recorded, all diamonds undergo a screening test on the ‘Diamond Sure’ instrument, which refers any diamonds which might be a Type IIa and Type 1aB for further testing. These diamonds are susceptible to colour enhancement by high pressure high temperature (HPHT) treatment and require further testing to determine whether their colour is still natural.

Improving colour
One of the most significant and successful treatments to improve the colour of a diamond is to subject it to high pressure and high temperature (HPHT) – a ‘permanent and irreversible’ treatment which caused uproar when it first emerged in March 1999. This treatment was developed by General Electric Co and significantly improved the colour of brown type II A diamonds to colourless, and to such an extent that they were ‘indistinguishable’ from natural colourless diamonds. The HPHT treatment also produced yellow and yellow-green colours in certain types of diamonds.

Colourless type IIA diamonds immediately became a cause of concern for the diamond market; were they treated or not? De Beer’s researchers worked feverishly and they pinpointed several characteristics to help to identify the HPHT treated diamonds by using photoluminescence (which is carried out by the sophisticated Raman Spectrometer) and absorption spectroscopy.

Despite their best efforts, no one has found a way to identify HPHT diamonds without the use of extremely sophisticated instruments which are only available in a very few laboratories worldwide. Fortunately for the UK trade, AnchorCert can provide this identification service, and regularly reports on large white type IIA diamonds.

Another interesting colour treatment is Serenity coated coloured diamonds. These diamonds are surface coated with a thin layer of foreign material to create the apparent colour. Demand for this treatment is due to the rarity of natural fancy-colour diamonds and their high price. This technique produces colours such as intense blue, green, yellow, and orange to pink to purple-pink. The coating can be identified by its gemmological, spectroscopic and chemical properties.

Magnification can help identify the coating by observation of interference-related colour film, scratches and patchy areas on the pavilion facets. The confirmation of the treatment can be done by using the UV-Vis to analyse the absorption spectra, and by chemical analysis of the thin film. The coating is a SiO2 film doped with Au or/and Ag for blue, pink and yellow colouration and a surface Fe2O3 film for orange colour. The colours produced by the treatment are stable for standard jewellery repair and cleaning procedures, but the treatment is not considered to be permanent.

Improving apparent clarity
Make a stone look cleaner can hugely improve its attraction and therefore its apparent value. Laser drilling to remove inclusions has been a common practice for years, but is easily detectable as it leaves an obvious drill hole which acts as a conduit for a strong acid to bleach the material or to remove it all together. This may or may not improve the clarity grade but makes the stone appear better by making the dark inclusions look white.

However, the latest internal laser or ‘KM treatment’ does not show the traditional surface reaching drill hole. KM (meaning ‘special drill’ in Hebrew) laser treatment uses diamonds with dark inclusions near the surface and causes a small cleavage to develop or expand around the inclusion.

Once the cleavage reaches the surface (now termed as feather) it acts as a conduit for acid to bleach the dark inclusion. Irregular wormlike channels are used to widen the cleavage to facilitate the entry of the bleaching solution. An experienced gemmologist can detect this treatment with a powerful microscope. Identification requires careful examination using different lighting techniques, but the treatment can be recognised by the presence of a transparent, mirror-like feather that contains unnatural-looking irregular channels and a connection of the internal inclusion to the surface of the stone.

Anu says: “All these treatments are acceptable in the diamond industry as long as they are disclosed at every level. We are proud that AnchorCert has invested in our training and the instruments to provide the UK trade with a team able to detect these treatments.”