HPHT (High Pressure High Temperature)

HPHT — High Pressure High Temperature — is the original synthetic diamond growth method, in commercial use since the 1950s. It replicates the conditions under which natural diamond forms in the Earth's mantle: roughly 5 to 6 gigapascals of pressure and 1,300 to 1,600 degrees Celsius, with diamond growing on a seed crystal from carbon dissolved in a metal solvent (typically iron, nickel, or cobalt).

The growth chamber is a high-pressure press — historically the BARS (split-sphere) press developed at the Soviet Academy of Sciences in the 1980s, more recently the cubic press developed by Chinese manufacturers. The press squeezes a small reaction cell containing a carbon source, a metal solvent, and a seed crystal to the target pressure, then heats it to the target temperature. Carbon dissolves into the metal solvent, migrates through the liquid metal toward the cooler seed crystal, and crystallizes onto the seed. A typical HPHT run takes 7 to 14 days to produce a finished crystal yielding a 0.5 to 3 ct polished stone.

HPHT-grown stones can be Type Ia (yellow body color from nitrogen incorporation), Type Ib (also yellow but from single-atom nitrogen), or Type IIa (colorless, from synthesis runs with deliberate nitrogen suppression). Colored HPHT lab-grown (fancy yellow, fancy orange) is straightforward; colorless HPHT requires nitrogen-getter additives in the reaction cell and is more expensive to produce.

HPHT has two distinct uses in modern lab-grown production. First, as the original synthesis route for colorless and colored lab-grown — though largely displaced by CVD for new gem production above 0.5 ct. Second, as a post-growth color treatment applied to CVD-grown stones to remove brown tint and shift color toward D-F. The "Post-growth treatment: HPHT" disclosure on IGI and GIA Laboratory-Grown Diamond Reports refers to this second use — the underlying stone was grown by CVD and then HPHT-treated to finish the color.

Industrial-grade HPHT diamond (grit, abrasive, cutting tools) is the largest commercial use of HPHT synthesis by volume; gem-quality HPHT is a smaller segment of the same industry.

HPHT origin is identified by DiamondView fluorescence (often blue or green, with octahedral or cuboctahedral growth sectors visible) and by characteristic metal-flux inclusions sometimes visible at 10×. The identification is reliable; HPHT-grown stones do not confuse with natural in routine grading.

HPHT lab-grown carries no premium over CVD lab-grown at equivalent grades. The market does not distinguish — both are "lab-grown" with growth method disclosed on the report.