HPHT laboratory-grown diamonds are grown within a reaction cell. These contain three key ingredients: a 'diamond seed', which is a tiny HPHT laboratory-grown diamond crystal, a mixture of three metals to make a catalyst, and graphite as the carbon source.

 

Under the high temperatures, the metal catalyst will melt. This dissolves the graphite releasing free carbon atoms into the molten metal which transports them to the base of the reaction cell. Due to the high temperatures and pressures, the carbon recrystallises as laboratory-grown diamond. This occurs on top of the seed, which acts as a nucleation point for growth - allowing one crystal larger to form.

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Note: The seed does not 'grow', but functions as a growth platform. The carbon bonds to it allowing larger crystals to grow rather than lots of little crystals (which producers do sometimes to grow the seeds themselves as well as industrial laboratory-grown diamond grit).

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Multiple stones of the same colour and quality can be produced within one reaction cell. All thats required are more 'seeds'. The amount of seeds possible depends on the size of the reaction cell, which, in turn, is determined by the type of HPHT machine being used. 

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Several machines have been designed to synthesis laboratory-grown diamonds using the HPHT method. The equipment must be able to impart the extreme pressures required so are often massive.

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The most common HPHT equipment is the 'cubic press' - a huge machine that utilises six hydraulic anvils to put pressure onto the reaction cell where the laboratory-grown diamond grows.

 

The cubic press was developed in China, who currently produce the majority of HPHT laboratory-grown diamonds.