High-pressure high-temperature (HPHT) treatment is a process by which a diamond's colour is permanently altered by subjecting it to conditions that approximate those of its original formation deep within the Earth — pressures exceeding 5 GPa and temperatures above 1,300 °C. The treatment can decolourise brownish diamonds to near-colourless or colourless grades, or, in certain structural configurations, produce vivid fancy colours including yellow, greenish-yellow, and blue. Because the change is permanent and the treated stone remains chemically and mineralogically a diamond, HPHT treatment occupies a uniquely sensitive position in disclosure law: it is neither a coating nor a filling, yet it materially alters the commercial value of the stone. Mandatory disclosure of HPHT treatment is now required under the trade standards of CIBJO, the US Federal Trade Commission (FTC), and the World Diamond Council, and its omission constitutes misrepresentation under consumer-protection legislation in most major jurisdictions.

Historical Background: The 1999–2000 Crisis

The commercial application of HPHT treatment to gem diamonds became publicly known in 1999, when General Electric and Lazare Kaplan International announced a process — marketed under the trade name Bellataire — capable of transforming Type IIa brownish diamonds into colourless or near-colourless stones. The announcement was preceded by a period during which treated stones had already entered the market without disclosure, creating significant alarm among dealers, graders, and consumers who had no reliable means of identifying them. The Gemological Institute of America (GIA) and other major laboratories responded rapidly, publishing detection protocols and establishing that certain spectroscopic signatures — particularly the modification of nitrogen-vacancy centres and changes in photoluminescence spectra detectable under laser excitation — could reliably distinguish HPHT-treated diamonds from untreated ones.

The episode exposed a structural vulnerability in the trade: because HPHT treatment leaves no obvious visual trace and because the resulting stone is not a simulant or a synthetic, existing disclosure frameworks were insufficiently specific. The industry's response was swift and, by the standards of gemstone commerce, unusually coordinated. Within two years, CIBJO had revised its Diamond Book to require explicit disclosure, the FTC had clarified its Guides for the Jewelry Industry to encompass permanent colour treatments, and the major grading laboratories had introduced dedicated notation on certificates.

The Regulatory Framework

Disclosure obligations for HPHT treatment now rest on several overlapping authorities:

• CIBJO (The World Jewellery Confederation): The Diamond Book — CIBJO's principal standard for diamond nomenclature and trade practice — requires that any diamond subjected to HPHT treatment be described as such at every point in the supply chain, from rough to retail. The term "treated" must appear in conjunction with the specific treatment type; generic phrases such as "enhanced" without further qualification are insufficient.
• US Federal Trade Commission: The FTC's Guides for the Jewelry, Precious Metals, and Pewter Industries (most recently revised in 2018) specify that it is deceptive to fail to disclose any material treatment that affects the value of a diamond. HPHT treatment is explicitly cited as an example of a treatment requiring disclosure. The FTC's standard applies to any seller marketing to US consumers regardless of where the transaction originates.
• World Diamond Council: The WDC's system of warranties, developed in the context of the Kimberley Process, incorporates disclosure of treatments as part of responsible trade practice, reinforcing the obligation at the wholesale and rough-trading level.
• National consumer-protection laws: In the European Union, the United Kingdom, Canada, Australia, and numerous other jurisdictions, the omission of a material fact that would affect a consumer's purchasing decision constitutes a misleading commercial practice under general consumer-protection statutes, independently of any gem-trade-specific regulation.

Laboratory Identification and Laser Inscription

The detection of HPHT treatment relies primarily on advanced spectroscopic techniques not available to the retail trade. The principal methods include:

• Photoluminescence (PL) spectroscopy: HPHT treatment modifies the population and configuration of nitrogen-vacancy (NV) centres within the diamond lattice. PL spectra collected at liquid-nitrogen temperatures reveal characteristic emission lines — notably the NV⁰ and NV⁻ centres — whose relative intensities and the presence or absence of associated defect lines allow trained gemmologists to distinguish treated from untreated stones with high confidence.
• Infrared absorption spectroscopy (FTIR): HPHT treatment converts aggregated nitrogen (Type IaAB) towards disaggregated or single-substitutional nitrogen (Type Ib or IIa), producing measurable changes in the infrared absorption spectrum. The ratio of A- to B-aggregate nitrogen, combined with the presence of H3 and H4 defect centres, provides corroborating evidence.
• DiamondView fluorescence imaging: Developed by the De Beers Diamond Trading Company, DiamondView uses short-wave ultraviolet illumination to reveal growth patterns and fluorescence distributions that can indicate HPHT treatment, particularly in stones treated to produce fancy colours.

Because these techniques require laboratory-grade instrumentation, reputable grading laboratories — including GIA, the International Gemological Institute (IGI), the Hoge Raad voor Diamant (HRD), and Gübelin Gem Lab — permanently laser-inscribe the girdle of any HPHT-treated diamond they certify, typically with the notation "HPHT Processed" or an equivalent phrase. This inscription, visible under 10× magnification, provides a durable, point-of-sale-accessible record that survives polishing, cleaning, and resetting. The inscription does not replace the grading report but serves as an independent, physical disclosure mechanism.

Disclosure Obligations Along the Supply Chain

The disclosure requirement is not limited to the retail point of sale. CIBJO and FTC standards both contemplate disclosure at each transfer of ownership, meaning that a wholesaler selling to a retailer, a cutter selling to a dealer, and a dealer selling to a consumer are each independently obligated to disclose. Invoice language, memoranda, and consignment agreements should all reflect the treatment status of any HPHT-treated stone. In practice, the grading certificate and girdle inscription together constitute the primary documentary record, but verbal or written disclosure at the point of negotiation is also expected.

Marketing materials — including website listings, auction catalogues, and printed advertisements — must describe HPHT-treated diamonds in terms that make the treatment status unambiguous. Phrases such as "colour-enhanced diamond" without specifying the treatment method are generally considered insufficient under current FTC guidance, which calls for disclosure that is clear and conspicuous.

Value Implications and Market Context

The commercial rationale for undisclosed HPHT treatment is straightforward: a colourless or near-colourless diamond commands a substantially higher price per carat than a comparable brownish stone. The price differential between a D-colour and an M-colour stone of equivalent cut and clarity can exceed several hundred per cent, and HPHT treatment can, in favourable cases, move a stone several grades up the colour scale. Disclosed HPHT-treated diamonds trade at a significant discount to untreated equivalents of the same apparent colour grade — a discount that reflects both the treatment itself and the market's preference for natural colour. This discount is well established and documented in trade pricing guides.

The existence of this price differential means that the incentive for non-disclosure remains present, and enforcement depends substantially on laboratory certification at the point of grading. Stones submitted to major laboratories without disclosure of prior treatment are identified through the spectroscopic protocols described above, and the resulting certificate will note the treatment regardless of the submitter's intentions. This creates a practical deterrent: a treated stone certified by a reputable laboratory will carry permanent documentation of its treatment status, making subsequent non-disclosure difficult to sustain.

Synthetic Diamonds and the Boundary of HPHT Disclosure

A separate but related disclosure issue concerns laboratory-grown diamonds produced by the HPHT method — the same high-pressure, high-temperature process used in synthesis since the 1950s. These stones are distinct from HPHT-treated natural diamonds and require disclosure as synthetic or laboratory-grown under both FTC and CIBJO standards. The two categories — HPHT-treated natural diamonds and HPHT-grown synthetic diamonds — are sometimes conflated in popular usage, but they are gemmologically and legally distinct. A natural diamond subjected to HPHT treatment retains its natural origin and must be disclosed as treated; a diamond grown by the HPHT process must be disclosed as laboratory-grown. Both disclosures are mandatory and non-negotiable under current standards.

Further Reading

• GIA Gems & Gemology — HPHT Annealing of Diamonds (Shigley et al., 1999)
• GIA Research: HPHT Treatment of Diamonds
• FTC Guides for the Jewelry, Precious Metals, and Pewter Industries
• CIBJO Diamond Book (current edition)