An anvil mark is a localised surface disruption found on diamonds that have undergone high-pressure, high-temperature (HPHT) treatment. During the HPHT process, a diamond is subjected to pressures exceeding 50,000 atmospheres and temperatures above 1,300 °C inside a hydraulic press; the stone rests in direct contact with the metal anvil components of that press. The resulting mechanical contact leaves characteristic abrasions, small pits, or zones of altered surface texture — collectively termed anvil marks — most commonly on the girdle or pavilion facets. Because these features arise specifically from the geometry and mechanics of industrial HPHT apparatus, their presence constitutes a treatment witness: physical evidence that the stone has been processed rather than left in its natural post-cutting state.

Formation and Appearance

The HPHT press applies load through a set of anvils — typically tungsten carbide or sintered ceramic — that grip the diamond within a pressure-transmitting medium. Even with carefully engineered cell assemblies, localised stress concentrations at the contact zones can abrade or pit the diamond's surface. The resulting marks may present as:

• Small, irregular pits or craters, sometimes with raised rims;
• Linear or curved scratches following the contour of the anvil face;
• Patches of disrupted polish with a matte or granular texture, contrasting with the surrounding mirror finish.

Under gemological microscopy, anvil marks are most readily observed on the girdle, where the stone is most directly gripped, though they may extend onto adjacent pavilion or crown facets depending on the press configuration. Their distribution is typically asymmetric and confined to discrete zones rather than uniformly distributed across the surface, which helps distinguish them from ordinary wear or post-treatment polishing damage.

Diagnostic Significance

The GIA documents anvil marks as one of several diagnostic indicators of HPHT treatment in diamond. They are rarely sufficient on their own to confirm treatment — a stone could theoretically acquire superficially similar surface damage through mishandling — but in combination with other HPHT indicators they form a compelling evidentiary picture. Corroborating features examined by laboratories include:

• Unusual graining or strain patterns visible under cross-polarised light;
• Infrared absorption spectra characteristic of type IIa or treated type Ia diamonds;
• Anomalous fluorescence or phosphorescence behaviour under ultraviolet illumination;
• Remnant graphite or flux inclusions within fractures or cavities that have been partially healed by the process.

When anvil marks are present alongside spectroscopic and optical evidence, major gemological laboratories — including the GIA, Gübelin Gem Lab, and SSEF — will note HPHT treatment on their reports. The GIA specifically uses the term anvil marks in its grading nomenclature for treated diamonds.

Trade and Disclosure Context

HPHT treatment is used commercially to improve the colour of diamonds, most commonly converting brownish or near-colourless stones to higher colour grades, or producing fancy colours such as vivid yellow or blue. Because the treatment can substantially alter a stone's apparent value, full disclosure is required under the trade standards of bodies including the AGTA and the World Jewellery Confederation (CIBJO). Anvil marks, as a detectable treatment witness, are one reason that HPHT treatment — unlike some other diamond enhancements — remains identifiable by qualified laboratories even after re-polishing attempts, since the marks may be partially removed but the spectroscopic signature typically persists. Buyers and valuers should request laboratory documentation for any diamond where HPHT treatment is suspected.

Further Reading

• GIA Gems & Gemology — HPHT Annealing of Diamonds
• GIA — Diamond Treatment Overview