Raman for HPHT Detection
Raman for HPHT Detection
Photoluminescence and Raman spectroscopy applied to the screening of HPHT-treated diamond
The application of Raman spectroscopy and photoluminescence (PL) analysis to the detection of high-pressure high-temperature (HPHT) treatment in diamond is one of the most consequential analytical capabilities in the modern gem laboratory. HPHT processing of brown type IIa diamond to produce colourless or near-colourless stones is an established commercial treatment, and the trade requires reliable laboratory tests to identify treated material. Raman and PL spectroscopy, conducted at low temperatures with specific laser excitations, provide the most sensitive non-destructive evidence available.
Why HPHT changes a diamond's optical signature
HPHT treatment of diamond involves heating the stone to roughly 2000 degrees Celsius under pressures of 5 to 7 gigapascals, conditions at which diamond is the thermodynamically stable carbon phase. The treatment dissociates nitrogen aggregates (especially the A-aggregate, two adjacent substitutional nitrogens), redistributes lattice vacancies, and modifies the population of defect centres responsible for visible-spectrum colour. In type IIa diamonds — those with negligible nitrogen — HPHT removes brown colour caused by lattice strain associated with plastic deformation, producing colourless or near-colourless stones from inexpensive brown rough.
The treatment also modifies the population of nitrogen-vacancy (NV) centres and silicon-vacancy (SiV) centres, both of which produce sharp luminescence lines under appropriate laser excitation. The diagnostic value of PL spectroscopy lies in the abundance, ratios, and detailed structure of these defect-centre emissions, which differ between untreated and HPHT-treated stones in patterns that the laboratory community has characterised over the past two decades.
Diagnostic features
The most widely cited PL features in HPHT detection include the NV0 centre at 575 nanometres and the NV- centre at 637 nanometres, both arising from a single substitutional nitrogen adjacent to a vacancy in two charge states. The H3 centre at 503 nanometres (two nitrogens flanking a vacancy) and the H4 centre at 496 nanometres are also informative; their ratios shift characteristically after HPHT processing. The SiV- centre at 737 nanometres, sometimes reported in the literature near 741 nanometres, is a marker of silicon incorporation common in CVD-grown synthetic diamond, and its appearance in PL spectra is one indicator of synthetic origin rather than HPHT treatment of natural material.
Identification typically requires liquid-nitrogen cooling of the sample to about 77 kelvin, which sharpens spectral lines and allows fine structural detail to emerge from the broad room-temperature emissions. Multiple laser excitations — commonly 514, 532, 633, and 785 nanometres — are used in sequence to populate different defect centres preferentially. Raman scattering from the diamond lattice itself, at 1332 wavenumbers, provides an internal calibration line and an indication of crystal stress.
Limits of the technique
Not every HPHT-treated diamond shows diagnostic PL features. Some treated stones lack the specific defect-centre fingerprints that allow confident attribution, and the laboratory community has documented cases where HPHT processing produces no readily detectable spectroscopic signature. In such cases, the report typically describes the limit of the analysis explicitly, noting that the spectroscopic data are inconclusive on the question of treatment.
Conversely, naturally occurring stones can show defect-centre populations that overlap the HPHT-treated range, particularly type IIa diamonds with substantial natural lattice strain. The interpretation of any single spectrum requires judgement informed by the type classification, the colour grade, the inclusion suite, and the broader analytical picture. The practice of HPHT detection is therefore one of pattern recognition across a multi-dimensional dataset rather than a binary test.
Laboratory practice
Major laboratories — GIA, IGI, HRD Antwerp, AGL, Gübelin, and SSEF — operate Raman-PL systems for HPHT screening on type IIa and other candidate stones. GIA's diamond grading reports identify HPHT processing on the report itself when detected, and the laboratory has published technical articles in Gems & Gemology describing the diagnostic protocols and the underlying defect-centre science.
For diamonds destined for major auctions and high-value private sales, HPHT screening is now part of the standard analytical workflow. The cost is modest relative to the potential value gap between treated and untreated stones at the high end, and the analysis is non-destructive, with the stone returned in the same condition in which it was received.
Distinction from synthetic-diamond detection
Raman-PL also plays a central role in the detection of synthetic diamond, both HPHT-grown and CVD-grown. The diagnostic features differ from those used for HPHT-treatment detection but share the same instrumental basis. Some HPHT-treated stones are also synthetic, and some CVD synthetics are subsequently HPHT-processed; the analytical workflow handles each case in turn rather than as a single test.
In the trade
For dealers and auction houses, the practical significance of Raman-PL is that the major laboratories' identification of HPHT treatment is reliable in the great majority of cases. Diamonds without HPHT processing certified by a major laboratory carry the standard market premium for natural untreated material. Stones identified as HPHT-treated trade at a substantial discount, and the trade rules of the major associations require disclosure on invoices and reports.