The Colour Change Processes in diamond

 

Gem-Quality Synthetic Diamonds

In 1970, General Electric announced a modified growth process that enabled them to produce large synthetic diamonds by the HPHT process. This technique is known as the ‘temperature gradient method’.

With this approach, the carbon source is placed near the middle of the growth capsule, and one or more synthetic diamond seeds are placed on a ‘seed pad’ at the bottom of the capsule. A substantial layer of the metal solvent-catalyst is placed between the carbon source and the seeds.

The temperature of the reaction volume is raised by a graphite heater sleeve surrounding the growth capsule (this is referred to as ‘indirect heating’). The bottom of the capsule is slightly cooler than the centre, and so there is a temperature gradient between the carbon source and the seeds. When the metal is melted, carbon dissolves at the central region of the capsule and moves down towards the seeds. Because the seed region is slightly cooler, diamond crystals begin to grow.

As before, this growth capsule is at the centre of a high-pressure press with about 55,000 atmospheres pressure applied. The process is sometimes referred to as the ‘reconstitution method’ because often the carbon source is synthetic diamond grit, which is relatively inexpensive. If graphite is used, there is a substantial reduction in the volume as the graphite is converted to diamond; however, if diamond grit is converted to large diamond crystals, there is no significant change in the volume of the growth capsule, and it is much easier to control and maintain the conditions necessary for high-quality growth.

If the growth process is extended to a few days, rather than a few minutes, large synthetic diamonds can be produced. The colour of the crystals is normally yellow to brown, again because of the presence of single nitrogen. However, as with the diamond grit, if the nitrogen is gettered, colourless crystals can be produced, and blue diamonds can be grown using nitrogen getters and adding boron.

The high-pressure presses (the so-called ‘belt’ presses) developed by GE, De Beers and others were very costly, and in the 1970s, it was uneconomic to grow gem-quality diamonds by the HPHT method. More recently, Russian researchers have developed the BARS1 apparatus, and this has been re-engineered by companies in the US, making it suitable for mass production. These presses have a small heated volume and can only produce one diamond per growth run. Nevertheless, following these developments, a number of groups have set up and are expanding factories with dozens of BARS machines, each capable of turning out a gem-quality diamond every few days.

Colourless diamonds are difficult to produce; the presence of small concentrations of nitrogen improves the crystalline properties of the diamonds and also results in a faster growth rate. For that reason, most of the current production of gem-quality synthetic diamonds is fancy yellow. Such diamonds can also be given a pink colour, using radiation and annealing.

At present, the annual production of synthetic diamond gemstones is only a few thousand carats, compared with 120 million carats of natural diamond mined each year. Amongst the natural diamonds, there is perhaps only one fancy coloured diamond for every 10,000 near-colourless ones. Consequently, in the future when a fancy yellow diamond, with a weight of up to 1.5 ct, or so, is encountered in the gem trade, there will be a high probability that it is synthetic.