Colour Enhancement by HPHT Annealing

 

Colour Enhancement by HPHT Annealing

The selling price of yellow diamonds varies with the depth of colouration. There is a similar curve for brown diamonds. For diamonds equal in all other respects, as the colour goes from very pale brown to a more notice- able brown the selling price drops, and for some attractive pronounced brown colours like ‘champagne’ and ‘cognac’, it may increase again. As before, the question arises: is it possible to take a cheap brown diamond, and improve its colour in some way, thereby enhancing its selling price? In particular, is it possible to take a brown diamond and remove the colour, converting the stone to a near-colourless D or E colour?

The answers to these questions again involved research by GE. In March 1999, the diamond gem trade was staggered by an announcement from Pegasus Overseas Ltd (POL) of Antwerp, Belgium, a subsidiary of Lazare Kaplan International (LKI), that they had begun marketing colour- enhanced diamonds. The diamonds had been processed by GE, and it was generally assumed, and subsequently confirmed, that the process involved treatment at high pressure and high temperature. Information circulating in the trade indicated that the starting material was brown type IIa diamond and that the GE process substantially reduced the brown colour.

The acronym GEPOL was soon attached to these colour-enhanced dia- monds. Shortly after the initial announcement, the diamonds were marketed in the USA under the trade name ‘Bellataire’. In March 2004, Littlejohn and Co., a private equity firm, announced the completion of its acquisition of the GE Superabrasives division from the GE Company. The company was renamed ‘Diamond Innovations’, and the transaction also included the gem technology behind the Bellataire diamond brand.

Prior to the revelation by GE in 1999, it had generally been thought that permanent colour changes to natural diamonds (produced by, for example, radiation and annealing) can only add to the pre-existing colour. Clearly, that is no longer the case. Later in the same year, Novatek, which manufac- tures high-pressure presses used in diamond synthesis, announced that they were also processing natural brown diamonds. Novatek set up a subsidiary company, NovaDiamond Inc., to produce and market these dia- monds. NovaDiamond did not pre-select the diamond type, and many of the stones they processed changed from brown to yellow/green.

 

Although these announcements were made in 1999, there was evi- dence a year or so earlier that similar colour changes were being pro- duced by laboratories in Russia. In retrospect, questions being raised in the mid-1990s about colour enhancement of natural brown diamonds probably indicate that these activities were already underway at that time.

In order to predict the colour change that will be produced in a brown diamond by HPHT processing, it is necessary to carefully characterize the stone. NovaDiamond did not have the facilities to do this, and are no longer producing significant quantities of colour-enhanced diamonds. However, in 2003, Sundance Diamonds, a division of U.S. Synthetic, announced the launch of its HPHT diamond-processing service for the diamond industry. (U.S. Synthetic is the world’s largest manufacturer of synthetic diamond components for use in oil and gas drilling.) To comple- ment the HPHT processing, Sundance has set up a characterization labora- tory to determine the suitability of stones for HPHT processing and to make a preliminary analysis of the colour change to be expected. They have demonstrated the capability of changing brown and cape diamonds to fancy colours, including yellow, orange and green, and of changing type II diamonds from unmarketable colours to colourless or near-colourless.

For any company that has invested in HPHT equipment, the annealing

of natural brown diamonds (which only takes a few minutes) can be much more profitable than growing gem-quality synthetic diamonds (which takes a few days). It is therefore not surprising that another manufacturer of synthetic diamond, Iljin of South Korea, announced in the summer of 2003 that they will be processing type I diamonds at HPHT conditions to produce fancy yellow, orange and greenstones. These diamonds are being marketed under the trade name ‘Nouv’.