HPHT pink refers to pink colour in diamond produced or modified by post-growth high-pressure, high-temperature treatment, sometimes followed by irradiation and low-temperature annealing. The treatment is an established commercial process, originally introduced for colourless and brown-to-colourless conversion in the late 1990s and extended in the 2000s into multi-step routes that target pink, red and purple-pink hues in suitable starting material.

Mechanism

Natural pink colour in diamond is associated with plastic deformation: glide planes through the lattice generate vacancies and nitrogen-vacancy defects whose absorption bands at around 550 to 570 nanometres produce a pink residual transmission. The natural process requires both nitrogen and a deformation history during the diamond's residence in the mantle, which is why fine pink diamonds are unusually rare.

HPHT treatment for pink colour uses a similar logic but drives it artificially. The starting material is typically a Type IIa or low-nitrogen Type IaB brown diamond, often material that lab analysis indicates has the appropriate defect chemistry. The stone is held at conditions within the diamond stability field, generally 5 to 7 GPa and 1800 to 2300 degrees Celsius, for periods of minutes to hours. The high temperature anneals out brown-causing defects and rearranges nitrogen and vacancies. In a single-step HPHT-only treatment the result on Type IIa material is usually colourless to faint pink. To produce a saturated pink, the HPHT step is followed by electron or gamma irradiation to introduce additional vacancies and a controlled low-temperature anneal in the 700 to 900 degree Celsius range, which mobilises vacancies into nitrogen-vacancy centres at the desired concentration.

Identification

HPHT-treated pink diamonds carry diagnostic features that competent laboratories can identify:

• Photoluminescence spectra showing characteristic NV0 (575 nm) and NV- (637 nm) centres at intensities and ratios that differ from those of natural pink diamonds, often with associated H3, H4 and 595 nm features.
• UV-visible-NIR absorption spectra showing the 550 to 570 nm band but accompanied by features inconsistent with a purely natural deformation history.
• Surface graphitisation or pitting on naturals, girdles or facets, sometimes visible at high magnification, where the stone was in contact with the cell wall.
• Frosted or hazy surface zones from the high-temperature exposure that have been re-polished but leave subtle remnants.
• Absent or reduced graining typical of natural plastic-deformation pinks, with the colour often more uniform and less zoned than nature would produce.

Disclosure and trade

The treatment is permanent for practical purposes; it does not fade in normal wear or light exposure. It is fully disclosable under FTC, CIBJO and major trade-organisation rules and must be reported on every laboratory document. GIA, IGI and other laboratories issue identification reports and treatment grading reports for HPHT-treated pink diamonds and apply girdle laser inscriptions where appropriate.

From a trade perspective HPHT pinks fill a market segment well below the price of natural pink diamonds, particularly for stones above one carat, where natural pink supply is constrained. The category should not be conflated with natural pinks, with HPHT-grown pink synthetic diamonds, or with surface-coated pink diamonds, all of which are separate identifications and price tiers.