HPHT Treatment of Natural Diamonds
HPHT can mean two entirely different things: a method of growing a new diamond and a subsequent treatment of an existing diamond. This chapter deals exclusively with the second case—a natural diamond that, after geological formation, is subjected to high pressure and high temperature to alter its defect structure and, most often, its color.
After treatment, such a stone remains a natural diamond with treated color. It does not become laboratory-grown.
Why the combination of pressure and temperature works
A natural diamond may contain vacancies, nitrogen aggregates, deformation-related defects, and other lattice imperfections that absorb part of the visible light.
Very high temperature allows certain defects to redistribute. High pressure helps keep the diamond stable during this thermal process and reduces the tendency toward conversion to graphitic carbon.
This does not mean that there is one HPHT “formula.” Conditions vary according to the objective, equipment, and starting material, and many commercial processes are not publicly documented at the level of a complete recipe.
The classic type IIa brown-to-near-colorless pathway
A commercial turning point came in 1999 with the appearance of GE POL/Bellataire treated natural diamonds. Particular attention was drawn to certain brown type IIa diamonds in which HPHT annealing could substantially reduce the brown component.
It is now clear that much of this brown color can be associated with plastic deformation and vacancy complexes. High-temperature reorganization can reduce the absorption responsible for the brown appearance.
It does not follow, however:
- that every brown type IIa diamond is a good candidate;
- that every type IIa diamond can become colorless;
- that every colorless type IIa diamond has been treated.
Type IIa is a screening context, not a treatment diagnosis.
Graining may remain after the color changes
HPHT may change the absorption system more than it changes the geometric history of deformation. Transparent graining, strain, or other traces of prior plastic deformation may therefore remain visible after the brown color has been reduced.
Such findings may be important, but they are not universal. Their absence does not exclude treatment, and their presence does not prove treatment without other data.
HPHT is not only “decolorization”
In other diamond types and defect systems, HPHT may produce yellow, greenish, blue, pink, or other results. Documented type Ia and IIb populations show that starting nitrogen, boron, vacancies, and prior treatments strongly influence the final result.
The formula “HPHT = colorless” must therefore not be used.
Detailed sequences involving irradiation and annealing remain for the following chapters.
What HPHT can do to inclusions
Extreme conditions may alter certain inclusions or fractures and locally cause graphitization or other thermal traces. Neither metal, graphite, nor a microscopic change, however, is a mandatory sign of treatment.
It is important to distinguish:
- a natural metallic inclusion;
- an HPHT growth remnant in a laboratory-grown diamond;
- a thermally altered inclusion in an HPHT-treated natural diamond.
One dark particle does not resolve that question.
Multimethod identification
A professional laboratory does not base its conclusion on one center or one image.
FTIR establishes diamond type and part of the defect structure.
UV-Vis-NIR reveals the absorption that shapes color.
Photoluminescence (PL) can detect very low concentrations of centers such as the NV systems.
Luminescence imaging provides spatial context.
Microscopy and strain may show remnants of the deformation history.
Detailed instrumental theory belongs in Chapters 66–72; what matters here is the convergence of evidence.
The historical 575/637 nm criterion
Early work on GE POL type IIa diamonds showed that the relationship between NV emissions near 575 and 637 nm could be a useful indicator of HPHT treatment in a particular population.
This is a historically important finding, but it is not a universal “575/637 test.” Fisher and Spits noted even in that early study that the relevant centers were not present in every treated stone.
More recent laboratory cases continue to use such PL relationships together with other characteristics, not in isolation.
Natural origin and treatment origin are two conclusions
A laboratory must first know that the stone is natural and then assess whether its color is the result of natural history or a subsequent HPHT process.
This is particularly important because HPHT also exists as a growth method for laboratory-grown diamonds and may be used as a post-growth treatment of CVD- or HPHT-grown material.
The term “HPHT” by itself, without a verb or context, is therefore not sufficiently precise.
Stability and disclosure
An HPHT color change is generally considered stable under normal wear conditions. This does not mean that a diamond as an object is invulnerable to heat, impact, or existing fractures.
As of August 7, 2026, GIA may issue reports for HPHT-processed natural diamonds with prominent disclosure of the treatment. In a documented GIA case from 2021, as well as in the Summer 2025 counterfeit-inscription case, after its post-treatment history was confirmed, the natural diamond with treated color received the girdle inscription TREATED COLOR. Such an inscription is an additional identification and disclosure layer, not a substitute for the laboratory conclusion itself.
Report policy is not a substitute for physically matching a stone to its document. Counterfeit inscriptions have also been documented on HPHT-treated natural diamonds, so an inscription always remains only one layer of identity.
Practical conclusion workflow
- confirm that the material is diamond;
- determine natural or laboratory-grown origin;
- establish diamond type;
- document bodycolor, graining, strain, and inclusions;
- analyze UV-Vis-NIR and FTIR;
- use PL and luminescence imaging as needed;
- compare the findings with known natural and treated populations;
- exclude alternative treatment pathways;
- give a treatment conclusion only on the basis of converging evidence;
- separately verify the report, inscription, and physical identity of the stone.
Chapter summary
- HPHT treatment of a natural diamond is not HPHT growth.
- A natural HPHT-treated diamond remains a natural diamond.
- HPHT works by reorganizing the defect structure under high temperature and pressure.
- Certain brown type IIa diamonds can become substantially lighter after HPHT treatment.
- Type IIa is not independent proof of treatment.
- Not all brown type IIa diamonds are equal candidates for decolorization.
- HPHT may produce other color outcomes, not only near-colorless.
- Transparent graining or strain may remain after the color changes, but they are not universal markers.
- Metal, graphite, or one microscopic change is neither a necessary nor sufficient treatment test.
- The 575/637 nm PL relationship is a historically important population indicator, not a universal verdict.
- Final identification requires a combination of FTIR, UV-Vis-NIR, PL, imaging, and gemological context.
- GIA can currently report HPHT-processed natural diamonds with clear treatment disclosure.
[VISUAL 60.1: Three HPHT contexts—HPHT-grown, natural HPHT-treated, and laboratory-grown post-growth HPHT-treated]
[VISUAL 60.2: Brown type IIa before/after—deformation defects, HPHT annealing, and reduced absorption]
[VISUAL 60.3: Screening is not diagnosis—type IIa, graining, strain, and PL as partial indicators]
[VISUAL 60.4: Multimethod workflow—natural origin → diamond type → UV-Vis-NIR/FTIR → PL/imaging → treatment conclusion]