The high temperature and high pressure synthesis method is also called the seed catalyst method. Graphite is a low-pressure stable phase, and diamond (the mineralogical name for diamond) is a high-pressure stable phase. The direct conversion from graphite to diamond requires very high pressure and temperature conditions, generally 10 GPa and a pressure and temperature above 3000 ℃ are required. If metal catalysts are involved (such as Fe, Ni, Mn, Co, etc. and their alloys), the temperature and pressure conditions required for graphite to become diamond will be greatly reduced, so the current high temperature and high pressure synthetic diamonds all have metal Catalyst participation. The metal catalyst as a solvent is between the carbon source (generally graphite) and the diamond seed crystal. The carbon source is at the high temperature end, and the seed crystal is at the low temperature end. Since the solubility of the carbon source at the high temperature end is greater than that of the low temperature end, the solubility difference caused by the temperature difference becomes the driving force for the diffusion of the carbon source from the high temperature end to the low temperature end. The seed crystal gradually precipitates, and the diamond crystal gradually grows. Since the driving force of crystal growth is caused by temperature difference, this method is also called temperature difference method.

HTHP synthetic diamonds are often cubes, octahedrons, and the combination of the two. The color is often yellow, yellowish brown, and the internal features such as colored bands and metal inclusions are common. The unmelted metal inclusions are needle-shaped, flake-shaped, and small columnar. Or irregular appearance with metallic luster, which makes synthetic diamonds magnetic.

Synthetic diamonds usually have no fluorescence under long-wave ultraviolet, and often have yellow, green-yellow, orange-yellow fluorescence under short-wavelength. Synthetic diamonds have characteristic banding under ultrashort-wave ultraviolet or cathode rays. Different growth areas show different colors of fluorescence band.

High-temperature and high-pressure synthetic diamonds (HPHT diamond) are mainly used in drilling (from oil and deep crust drilling to digging holes in teeth), cutting (from cutting marble to cutting gems), aerospace (from cabin windows to space probes), and electronics industry (from electronic instruments) To supercomputer chips), heat exchange (from large boilers to kitchen tableware) and jewelry and many other fields.

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