HPHT-grown colourless diamond is a laboratory-grown diamond produced by the high-pressure, high-temperature method and falling within the colourless to near-colourless grading range, conventionally D through K on the GIA scale. Colourless HPHT growth is technically more demanding than yellow growth: the cell must be configured with a nitrogen getter, typically titanium or aluminium, that captures nitrogen as it dissolves in the metal flux and prevents incorporation of nitrogen donors that would otherwise produce a yellow tint. The resulting crystals are predominantly Type IIa with very low nitrogen and very low boron, the same chemical class as the rarest natural colourless diamonds.

Growth conditions

The cell sits within the diamond stability field, with operating conditions typically near 5.5 GPa and 1350 to 1450 degrees Celsius. A diamond seed at the cooler end of a temperature gradient receives carbon precipitating from a saturated iron-cobalt or iron-nickel flux. Growth times for jewellery-sized rough commonly run from several days to a few weeks depending on size and the producer's economics. Cubo-octahedral morphology with prominent cube faces dominates, with growth-sector boundaries often visible in completed rough as faint colour zoning that survives into faceted stones.

Identification

Identification by a competent gemmological laboratory is reliable. The key indicators are:

• Cross-shaped or geometric strain patterns under crossed polarisers, in contrast to the irregular tatami-pattern strain of most natural Type IIa diamonds.
• Cubo-octahedral growth-sector zoning visible under deep ultraviolet imaging, including blue, green or yellowish fluorescence following the sector boundaries.
• Phosphorescence after short-wave ultraviolet excitation, often present even in stones that show little or no long-wave fluorescence to the naked eye.
• Photoluminescence spectra at liquid-nitrogen temperature showing characteristic peaks attributable to nickel-related defects and other growth-related centres.
• Occasional metallic flux inclusions visible at the loupe; these are magnetic and unique to HPHT growth.

FTIR spectroscopy confirms Type IIa character, but Type IIa alone is not diagnostic since natural Type IIa diamonds also exist; the distinguishing evidence comes from photoluminescence and ultraviolet imaging.

Market and disclosure

HPHT-grown colourless diamonds compete with chemical-vapour-deposition material in the colourless laboratory-grown segment. HPHT material historically dominated the smaller-size and melee markets while CVD captured larger sizes; that division has eroded as HPHT producers have scaled up. Wholesale prices have fallen sharply since 2018 and continue to track downward as production capacity expands, particularly in India and China.

Disclosure is mandatory. GIA, IGI, HRD and other major laboratories issue distinct laboratory-grown diamond reports and apply a laser inscription to the girdle of every faceted stone. The Federal Trade Commission's revised Jewelry Guides allow the term "laboratory-grown diamond" or equivalent and prohibit any presentation that implies a stone is natural when it is not. CIBJO's Diamond Blue Book carries similar requirements and is the reference document for European trade. The trade should not use "cultured" or "cultivated" without immediately qualifying the stone as laboratory-grown.