High Pressure High Temperature (HPHT) is a way in which synthetic diamonds are manufactured. HPHT uses what are called “presses” in order to recreate the environment inside of the Earth. The two main presses used are belt presses and cubic presses.
The original HPHT design was invented by H. Tracy Hall. It uses a belt press. A cylindrical volume is used. Anvils are placed on the top and bottom which provide both heat and pressure. In order to keep the pressure where it is needed, a belt made out of pre-stressed steel is used. Sometimes a belt press uses hydraulic pressure as opposed to a steel belt. However, both operate under the same principals. Belt presses were the original way HPHT devices were used. They operate in the same fashion that they did when first invented, except on a much larger scale.
Cubic presses use a cube-shaped volume. It uses multiple anvils to put pressure on the volume. Originally only four anvils were used, however, cubic presses now use six anvils because it maximizes the pressure available. Cubic presses are smaller than belt presses, but they are not able to be made into large scale versions like belt presses are. On the other hand, belt presses are able to create higher temperatures and greater pressure which allow synthetic diamonds to be produced quicker than a cubic press.
HPHT is the most commonly used system for producing synthetic diamonds, mainly because of its low cost.
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