How to make a diamond

Research commissioned by the Diamond Trading Company shows that 94% of women prefer natural diamonds to synthetic ones.

However for the remaining 6% there are two established ways to make your own diamond.

The first way, first achieved by Swedish company ASEA in 1953, and then by GEC in the US involves using large pressures and temperatures. Synthesis takes place within a cylindrical capsule containing a source of carbon, a solvent catalyst made from cobalt, nickel and iron, and a seed crystal. The capsule is placed between an anvil and die made from tungsten carbide at a pressure between 5 GPa and 7.1 GPa (50 000 to 70 000 atmospheres) and temperature between 1200 and 1500 ^oC. A temperature gradient of a few tens of degrees can help growth from the seed diamonds. These high pressure high temperature (HPHT) techniques can grow diamonds from a few hundredths to a few tens of carats. With the largest commercially available diamond being 3 carots (0.6 g).

Uncut HPHT diamonds differ from natural diamonds in there external shape. HPHT diamonds have a cubo-octahedral morphology as opposed to the octahedral morphology of natural diamonds. After cutting the original shapes can be distinguished because the uptake of impurities is different in different growth directions. Individaul faces exhibit different colours or fluoresence intensities. Some HPHT diamonds will exhibit strong fluorescence due to electronic transitions at atomic impurities containing nickel or cobalt from the solvent during growth. Inclusions of titanium or zirconium metal can also be trapped in the diamond when they are added to limit discolouration of the diamond by the prescence of nitrogen.

Chemical vapour deposition (CVD)
Another way of making diamond is to form a plasma from carbon-containing gas which is then deposited onto wafers of synthetic diamond. Carbon is initially deposited as graphite, but hydrogen etches away any graphite that is formed during deposition leaving only diamond structures behind. The ability to produce extermely high-purity diamonds with well-controlled doping makes this the preferred route to explore future technical applications such as electronic devices.

Links:
Transcript of the Diamond Makers, 1999 BBC Horizon