LPHT, an abbreviation for low-pressure high-temperature, is a category of laboratory treatment applied to diamonds at temperatures above approximately 1800 degrees Celsius but at pressures only modestly elevated above atmospheric, often within a non-oxidising gas environment. The technique is distinguished from HPHT (high-pressure high-temperature) treatment, in which both pressure and temperature are raised into the diamond-stable region of the carbon phase diagram. LPHT processing is most often discussed in connection with the colour modification of CVD-grown synthetic diamond, although it has also been investigated for natural stones.

Mechanism

At sufficient temperature, point defects within the diamond lattice gain enough thermal energy to migrate, anneal, or recombine. In CVD diamond as grown, the dominant colour-causing centres are typically nitrogen-vacancy (NV) and silicon-vacancy (SiV) complexes, together with isolated nitrogen and other intrinsic defects, producing brown or pink-brown bodies. LPHT annealing allows nitrogen and vacancies to redistribute, often suppressing the brown component and shifting the apparent colour toward near-colourless or, with controlled defect engineering, into stable pink. The pressure regime, while well below diamond stability at the relevant temperature, is sufficient to inhibit graphitisation when combined with a suitable gas atmosphere.

Industrial application

LPHT has become a standard post-growth treatment in the CVD synthetic diamond industry, applied to a substantial proportion of gem-quality CVD output before cutting. The treatment is sometimes left undeclared by individual sellers but is required by the GIA and IGI to be disclosed on grading reports for synthetic diamonds. Because the structural changes induced by LPHT can mimic features once thought diagnostic of natural diamond, gemmological screening in the post-LPHT era relies more heavily on photoluminescence at low temperature than on simple absorption-spectrum interpretation.

Detection

Identification of LPHT-treated CVD diamond rests on a combination of growth-related strain patterns visible under cross-polarised light, photoluminescence peaks (notably the silicon-vacancy doublet near 736.6/736.9 nm), and the absence or alteration of brown-related absorptions. The DiamondView instrument, developed by De Beers and now standard in major gem laboratories, reveals layered growth fluorescence characteristic of CVD synthesis regardless of whether the stone has been LPHT-annealed. GIA reports CVD diamonds with the qualifier treated colour when LPHT or other post-growth treatments have demonstrably been applied.

In short, LPHT is best understood as a routine but disclosable step in the production chain for laboratory-grown diamond, comparable in role to heating and oiling in the coloured stone trade. Its detection, like that of HPHT in natural stones, has driven significant advances in laboratory instrumentation over the last decade.