A HPHT diamond begins as a small diamond seed placed in carbon. Both are exposed to temperatures of around 1,500 °C and pressurised to approximately 1.5 million pounds per square inch. The carbon melts and starts to form a diamond around the starter seed, before being cooled to form a pure carbon diamond.
A CVD diamond begins as a slice of diamond seed, which is placed in a chamber and heated to 800°C. The chamber is filled with gases, which are are ionised into plasma using technology similar to that of microwaves or lasers. The ionisation breaks the gases' molecular bonds and the carbon adheres to the diamond seed before crystallising.