gemmology.dev – HPHT Diamond Treatment: Deep Diagnostic Reference

Process and Conditions

High Pressure High Temperature (HPHT) annealing subjects natural diamonds to
5–6 GPa and 1700–2100 °C, replicating mantle formation conditions. Diamonds are
encapsulated in a metal capsule (typically Fe or Co alloy) to prevent graphitisation,
and the treatment modifies nitrogen defect centres in ways that depend on diamond type.
The commercially most important application is decolourising brown Type IIa diamonds
to near-colourless (D–H range): Type IIa stones lack the N3 nitrogen aggregates
responsible for the blue LWUV fluorescence seen in ~80% of gem-quality natural
diamonds, so absence of that blue glow is the primary screening trigger for HPHT
treatment ^[1]. Spectroscopic analysis of GE POL HPHT-annealed
Type IIa stones shows that any residual yellow tint results from low concentrations
of single nitrogen not seen in untreated diamonds ^[1]. Definitive
identification requires FTIR, photoluminescence spectroscopy at 77 K, and DiamondView
imaging; standard gem-testing instruments alone are insufficient. A thorough grasp of
Understanding the diamond type system is required for interpreting these results ^[2].

Outcomes by Diamond Type

HPHT Treatment: Mechanisms and Results by 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) ^[2].
HPHT-treated type IIa diamonds frequently show no blue LWUV fluorescence, which is anomalous
for gem-quality colourless stones. This absence is the primary screening trigger
^[1] ^[3].

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

HPHT Diamond Detection (simplest to most advanced)

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 is issued 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

HPHT Diamond Treatment: Deep Diagnostic Reference