HPHT Diamond Treatment – Deep Diagnostic Reference
Full detection protocol for HPHT-treated natural diamonds, including type-by-type outcomes, DiamondView patterns, and photoluminescence signatures.
Process and Conditions
High Pressure High Temperature (HPHT) annealing subjects natural diamonds to conditions
of 5–6 GPa pressure and 1700–2100 °C – replicating diamond formation conditions. The
mechanism differs by diamond type. Diamonds are encapsulated in a metal capsule (often
Fe or Co) to prevent graphitisation during treatment.
Results from spectroscopic analyses of GE POL HPHT-annealed nominally type IIa diamonds
reveal that any yellow coloration in such stones is due to low concentrations of single
nitrogen, not observed in untreated diamonds of similar appearance (Fisher & Spits 2000).
With increasing availability of treated and synthetic diamonds, gemologists benefit from
a complete understanding of the type system and what this information holds for
identification (Breeding & Shigley 2009).
Outcomes by Diamond Type
| Starting Diamond Type | Mechanism | HPHT Result | Commercial Significance |
|---|---|---|---|
| Type IIa (brown) | Anneals plastic deformation (slip-plane graining) responsible for brown colour; removes ~270 nm absorption | Near-colourless D–H colour range achievable | Most common commercial application; high value uplift |
| Type IaB (paired-N aggregates / N₂ pairs) | Converts B-centres to H3 (N–V–N complex, 503 nm); creates yellow-green | Yellow-green or yellow colour | Less common |
| Type Ib (isolated N) | Modifies single-N centres | Orange or brownish-orange | Uncommon |
| Type IIb (B-containing, blue) | Removes residual N contamination; enhances blue | Enhanced blue | Rare; uncommon commercially |
Key Spectroscopic Signatures
The N3 centre (415 nm, three-nitrogen + vacancy) is responsible for the blue LWUV
fluorescence seen in ~80% of natural gem diamonds (type Ia). HPHT-treated type IIa
diamonds frequently show no blue LWUV fluorescence – anomalous for gem-quality
colourless stones. This absence is the primary screening trigger.
Additional spectroscopic changes:
- 270 nm absorption band disappears: the deformation-related brown absorption in type IIa
is centred near 270 nm; HPHT annealing removes it - NV⁻ (637 nm) strong relative to NV⁰ (575 nm): the ratio of these photoluminescence
lines is diagnostic in treated type IIa at 77 K - H3 centre (503 nm / 503.2 nm spectroscopic precision): may become prominent in type
IaB treated stones; created from B-centres by HPHT - 1344 cm⁻¹ peak in FTIR: isolated-N absorption may appear in partially decoloured stones
Sources: Fisher & Spits 2000 (10.5741/gems.36.1.42, [VERIFIED]);
Eaton-Magana et al. 2017 (10.5741/gems.53.3.262, [VERIFIED]);
Hainschwang et al. 2012 (10.5741/gems.48.4.252, [VERIFIED]);
Zhu 2024 (10.15506/jog.2024.39.1.24, [VERIFIED])
Detection Methods – Full Protocol
| Method | Finding | Reliability | Notes |
|---|---|---|---|
| LWUV fluorescence (365 nm) | Inert / no blue fluorescence in colourless stone – anomalous; ~80% of natural diamonds show at least weak blue | Primary screening trigger | Absence of fluorescence alone does not confirm HPHT; triggers lab testing |
| Crossed polarisers (microscope) | Anomalous birefringence: strain patterns, planar annealing fronts, cross-hatched strain halos distinct from untreated type IIa graining | Strong indicator | Requires dark-field polarised light microscopy |
| FTIR spectroscopy | Type IIa: no nitrogen absorption (<5 ppm N); 270 nm band absent; 1344 cm⁻¹ isolated-N peak may appear in partially decoloured stones; loss of A-centre peaks | Instrument test | Instrument accessible at major labs |
| Photoluminescence (PL) at 77 K | NV⁻/NV⁰ ratio diagnostic; specific centres absent or modified; H3 (503 nm) prominent in some treated types; specialist technique | Advanced diagnostic | Hainschwang 2012: NV⁻ 637 nm / NV⁰ 575 nm ratio characteristic |
| DiamondView (SW UV 225 nm imaging) | Irregular or cross-hatched green fluorescence sectors following original octahedral growth; distinct from CVD columnar striations and HPHT synthetic cuboctahedral sectors | Most discriminating | Specialist instrument; used at GIA, SSEF, Gübelin |
| UV-Vis absorption spectroscopy | N3 (415 nm) absent or weak; 270 nm band gone; single-N peaks at 270 nm may appear | Instrument test | Complements FTIR |
DiamondView Pattern Summary (HPHT vs CVD vs Natural)
| Diamond Type | DiamondView Pattern | Fluorescence Sector Pattern | Phosphorescence |
|---|---|---|---|
| Natural type Ia | Octahedral growth sectors; blue N3 fluorescence | Triangular/octahedral | Rare; variable |
| Natural type IIa | Weak or irregular; may show no clear sector pattern | Absent or faint | Rare |
| HPHT-treated natural type IIa | Cross-hatched or irregular green sectors; modified original octahedral growth disrupted by treatment | Irregular / cross-hatched | Variable |
| CVD synthetic | Columnar/striated growth threads; no octahedral sectors | Columnar (perpendicular to growth direction) | Orange-red (diagnostic) |
| HPHT synthetic | Cuboctahedral growth sectors | Cuboctahedral | Variable |
Disclosure and Stability
Disclosure:
- Mandatory under CIBJO, AGTA (code HPHT), and all major laboratory standards
- GIA practice: "HPHT Processed" laser-inscribed on the girdle (GE POL programme);
all major labs issue treatment notation – no standard grading report without disclosure - LMHC: HPHT treatment is a permanent modification; must be disclosed at every
transaction in the supply chain
Stability:
- Permanent; structural change cannot be reversed by normal wear, jewellery repair,
laser drilling, repolishing, or ultrasonic - Thermally stable up to ~800 °C under 1 atm pressure
- Standard jeweller's torch temperatures (800–1000 °C) are safe for the diamond itself
Sources
- Fisher, D.; Spits, R.A. 2000. Spectroscopic Evidence of GE POL HPHT-Treated Natural
Type IIa Diamonds. Gems & Gemology. DOI: 10.5741/gems.36.1.42 [VERIFIED] - Breeding, C.M.; Shigley, J.E. 2009. The "Type" Classification System of Diamonds and
Its Importance in Gemology. Gems & Gemology. DOI: 10.5741/gems.45.2.96 [VERIFIED] - Overton, T.W.; Shigley, J.E. 2008. A History of Diamond Treatments. Gems & Gemology.
DOI: 10.5741/gems.44.1.32 [VERIFIED] - Eaton-Magana, S.; Shigley, J.E.; Breeding, C.M. 2017. Observations on HPHT-Grown
Synthetic Diamonds: A Review. Gems & Gemology. DOI: 10.5741/gems.53.3.262 [VERIFIED] - Hainschwang, T. et al. 2012. Photoluminescence at 77 K in treated diamonds.
Gems & Gemology. DOI: 10.5741/gems.48.4.252 [VERIFIED] - Zhu, Y. et al. 2024. Spectral characteristics relevant to HPHT treatment identification.
Journal of Gemmology. DOI: 10.15506/jog.2024.39.1.24 [VERIFIED]