Post-growth HPHT treatment

Post-growth HPHT treatment is a high-temperature anneal applied to a finished CVD lab-grown diamond to remove the brown carbon-vacancy tint that forms during CVD growth and shift the body color toward the colorless D-F range. The treatment is applied after the CVD growth run is complete and the rough stone has been cut from the growth crystal; the finished stone is heated to roughly 2,000 degrees Celsius under pressure (the same HPHT press used for HPHT synthesis but for a shorter cycle) for several hours.

The mechanism is annealing — heating the diamond to a temperature where vacancy defects in the carbon lattice can migrate, recombine with carbon atoms, and eliminate the absorption pattern that produced the brown tint. The result is a colorless stone that started its life as a brown CVD-grown crystal. The treatment is stable, undetectable to the eye, and required to be disclosed on every IGI and GIA Laboratory-Grown Diamond Report as "Post-growth treatment: HPHT."

Most colorless CVD lab-grown on the market has received post-growth HPHT. The reason is economic: CVD growth optimised for speed produces brown tint in most runs, and post-growth HPHT is the cheapest path from brown CVD to D-color CVD. As-grown D-color CVD (no post-growth treatment) requires slower growth conditions and tighter nitrogen control during synthesis, which costs more. The trade has not assigned a meaningful premium to as-grown CVD over post-growth-HPHT-treated CVD — both grade as D-F colorless and both are correctly disclosed.

The treatment also affects fluorescence behavior. Post-growth HPHT can shift CVD fluorescence patterns under DiamondView from orange/red (as-grown CVD) toward weaker or differently-colored responses; GIA and IGI grading accounts for this in the identification process. The identification of CVD origin remains reliable even after post-growth HPHT — the underlying growth signatures (FTIR peaks, microscopic growth features) survive the anneal.

For comparison, natural diamond can also receive HPHT treatment to remove brown body color and shift toward D-F. The result on a natural is disclosed on a GIA Diamond Grading Report as "HPHT processed" with a laser inscription on the girdle marking the treatment. HPHT-treated naturals trade at 30% to 50% discount to untreated naturals at the same finished color. Post-growth-HPHT lab-grown does not carry an equivalent discount within the lab-grown market — the treatment is the default, so the "no post-growth treatment" claim is the premium tier.

The disclosure language matters. "HPHT" on a natural report means treated natural. "Post-growth HPHT" on a lab-grown report means CVD-grown then HPHT-finished. The two are different.