HPHT

HPHT stands for high-pressure high-temperature. It is a method for producing synthetic diamonds by replicating the conditions under which natural diamonds form deep in Earth's mantle. In HPHT growth, carbon sources and a metal catalyst are subjected to high pressures and temperatures inside specialized presses, enabling carbon to crystallize on a diamond seed.

History and development: The technique was developed in the mid-20th century, with early breakthroughs in the 1950s by industrial laboratories including General Electric. Over subsequent decades, refinements improved crystal quality and production efficiency, leading to commercial use for both industrial and gem-quality diamonds.

Process details: Typical pressures range from about 4.5 to 6 gigapascals (GPa) and temperatures around 1300–1600°C. The reactor uses a carbide or cubic press (or belt press) containing a metal solvent/catalyst such as iron–nickel–cobalt alloy. A seed crystal sits in contact with the carbon source; under these conditions carbon dissolves in the metal and precipitates as diamond on the seed, promoting growth. The process can be adjusted to influence crystal size, quality, and color.

Applications and color treatment: HPHT diamonds can be gem-quality or industrial-grade. The method is also used to alter the color of some brown diamonds (producing colorless or fancy colors) and to improve clarity by repositioning inclusions. HPHT is one of the main methods for synthetic diamond production alongside chemical vapor deposition (CVD).

Detection and market: Gemological laboratories distinguish HPHT diamonds from natural stones using spectroscopy and inclusion analysis. Market disclosures vary by jurisdiction, but HPHT and CVD diamonds are widely traded as synthetic diamonds.