HPHT synthetic diamond is a diamond grown in a laboratory press by replicating temperatures and pressures within the diamond stability field, typically 5 to 6 GPa and 1300 to 1500 degrees Celsius. The process was developed independently in the 1950s by ASEA in Sweden and General Electric in the United States, with the first reproducible single-crystal jewellery-grade growth published by Sumitomo Electric in the 1980s. HPHT remains, alongside chemical-vapour deposition, one of the two commercial routes to laboratory-grown diamond.

How it works

The cell is built around a metal solvent, typically iron-cobalt or iron-nickel, that dissolves carbon at high temperature. A diamond seed sits at one end of the cell and a graphite source at the other; a temperature gradient drives carbon to migrate through the molten flux from the hot graphite to the cooler seed, where it precipitates as diamond. The whole assembly is loaded into a large multi-anvil press, a belt press or a cubic press; production-scale facilities use cubic presses operating in continuous campaigns lasting days to weeks per cycle.

The growth conditions favour a cubo-octahedral crystal habit with prominent cube faces, in contrast to the dominant octahedral habit of natural diamonds formed over geological time. Dopants and getters are added to the cell to control colour: a nitrogen getter such as titanium or aluminium captures nitrogen and produces near-colourless Type IIa output; allowing nitrogen to remain produces yellow Type Ib stones; adding boron produces blue Type IIb stones.

Identification

HPHT synthetic diamonds are routinely identified by major laboratories. Diagnostic features include:

• Cubo-octahedral growth-sector zoning, often visible as cross-shaped fluorescence patterns under deep ultraviolet imaging instruments such as the DiamondView.
• Metallic flux inclusions of iron-cobalt or iron-nickel alloy, which are magnetic and which natural diamonds do not host.
• Phosphorescence after short-wave ultraviolet excitation, a useful screening flag for laboratory-grown origin.
• FTIR spectra confirming Type Ib, IIa or IIb character consistent with the dopant configuration of the growth cell.
• Photoluminescence spectra at low temperature showing characteristic nickel- and silicon-related centres tied to the metal flux composition.

Screening instruments at the dealer and laboratory level, including GIA's iD100 and the De Beers SynthDetect, identify HPHT synthetic material reliably for round brilliants and most fancy shapes within the colourless to near-colourless range.

Trade context

HPHT-grown diamonds are real diamonds with the same hardness, refractive index, dispersion and density as natural diamond. They must be disclosed at every point of sale under FTC, CIBJO and major trade-organisation rules, using terms such as "laboratory-grown", "laboratory-created" or "synthetic" diamond. Wholesale prices for HPHT colourless material have fallen substantially since 2018 and continue to decline as production capacity expands.

For the working dealer the practical points are that HPHT synthetics share their physical and optical properties with natural diamond, that disclosure is non-negotiable, and that a current report from a recognised laboratory is the only defensible basis for trading the stones at value.