HPHT treated

The caption of each scene in the animation above (the first scene lists its three panel captions), with sources.

1. Scene 1HPHT treatment (brown type IIa diamond as the example)

   Before: The starting material for HPHT decolorizing is mainly natural brown type IIa diamond.

   During: The diamond goes into the same type of high-pressure apparatus used to grow single-crystal synthetic diamond and is heated under pressure.

   After: Decolorized diamonds often retain clearly visible transparent graining that was once brown.

   Treated zone: colour-causing defects inside the diamond

   Sources

   • Observations on GE-Processed Diamonds: A Photographic Record, by Thomas M. Moses et al. (Gems & Gemology, Fall 1999, Vol. 35, No. 3)
   • GE POL Diamonds: Before and After, by Christopher P. Smith et al. (Gems & Gemology, Fall 2000, Vol. 36, No. 3)
   • Identification of HPHT-Treated Yellow to Green Diamonds, by Ilene M. Reinitz et al. (Gems & Gemology, Summer 2000, Vol. 36, No. 2)
   • Natural-Color D-to-Z Diamonds: A Crystal-Clear Perspective, by Sally Eaton-Magaña et al. (Gems & Gemology, Fall 2020, Vol. 56, No. 3)
   • A History of Diamond Treatments, by Thomas W. Overton and James E. Shigley (Gems & Gemology, Spring 2008, Vol. 44, No. 1, pp. 32–55)

2. Scene 2Process conditions

   The diamond goes into the same type of high-pressure apparatus used to grow single-crystal synthetic diamond and is heated under pressure. GE's exact decolorizing conditions are proprietary; a 2000 report on treating nitrogen-bearing type Ia brown diamonds to yellow-green gives a confining pressure of about 6 GPa and temperatures up to 2100°C for short periods (less than 30 minutes, in some cases much less), and says these are similar to GE's decolorizing conditions. A 2020 paper says most commercial HPHT treatments are conducted in the 1900–2100°C range. The high pressure is there to suppress graphitization at these temperatures; but per the 2000 report, above 1850°C at about 6 GPa is in the graphite stability zone, and the transformation does not happen instantly in bulk: crystal and fracture surfaces are affected first, so the stone must be repolished afterwards.

   Sources

   • Identification of HPHT-Treated Yellow to Green Diamonds, by Ilene M. Reinitz et al. (Gems & Gemology, Summer 2000, Vol. 36, No. 2)
   • GE POL Diamonds: Before and After, by Christopher P. Smith et al. (Gems & Gemology, Fall 2000, Vol. 36, No. 3)
   • Natural-Color D-to-Z Diamonds: A Crystal-Clear Perspective, by Sally Eaton-Magaña et al. (Gems & Gemology, Fall 2020, Vol. 56, No. 3)
   • A History of Diamond Treatments, by Thomas W. Overton and James E. Shigley (Gems & Gemology, Spring 2008, Vol. 44, No. 1, pp. 32–55)

3. Scene 3How it works

   Per modern work, the vast majority of brown diamonds owe their colour to vacancy clusters, groups of missing carbon atoms created by natural plastic deformation, generally thought to contain about 40–60 vacancies each. HPHT annealing dissociates the clusters and the brown disappears. The graining and strain left by the deformation remain essentially unchanged after treatment, so it is not the long-range lattice distortion that is annealed out; the 2000 paper suggested the brown was linked to defects attached to dislocations and called the exact mechanism unknown.

   Sources

   • Natural-Color Pink, Purple, Red, and Brown Diamonds: Band of Many Colors, by Sally Eaton-Magaña et al. (Gems & Gemology, Winter 2018, Vol. 54, No. 4)
   • GE POL Diamonds: Before and After, by Christopher P. Smith et al. (Gems & Gemology, Fall 2000, Vol. 36, No. 3)
   • A History of Diamond Treatments, by Thomas W. Overton and James E. Shigley (Gems & Gemology, Spring 2008, Vol. 44, No. 1, pp. 32–55)

4. Scene 4After treatment and how it is detected

   The loss of colour comes from a reduction of the absorption continuum and of the general absorption level in the visible to ultraviolet. Decolorized diamonds often retain clearly visible transparent graining that was once brown. The samples studied were D–H after processing, though not all stones will achieve such results; the colour is permanent to normal jewellery manufacturing, wear and repair.

   Standard gemological testing has difficulty identifying it, and definitive identification needs a properly equipped gemological laboratory: detecting HPHT treatment in a type IIa diamond generally requires absorption and/or photoluminescence spectra taken with the diamond cooled to a low temperature. Possible signs include graphitized inclusions surrounded by a halo of tiny radiating cracks, partially healed fractures, and a frosted look from etching or pitting, or grey to black graphitization, where naturals or fractures reach the surface; these are a good indication of treatment but not proof by themselves.

   Sources

   • GE POL Diamonds: Before and After, by Christopher P. Smith et al. (Gems & Gemology, Fall 2000, Vol. 36, No. 3)
   • Natural-Color D-to-Z Diamonds: A Crystal-Clear Perspective, by Sally Eaton-Magaña et al. (Gems & Gemology, Fall 2020, Vol. 56, No. 3)
   • A History of Diamond Treatments, by Thomas W. Overton and James E. Shigley (Gems & Gemology, Spring 2008, Vol. 44, No. 1, pp. 32–55)
   • Gemstone Enhancement and Its Detection in the 2000s, by Shane F. McClure, Robert E. Kane and Nicholas Sturman (Gems & Gemology, Fall 2010, Vol. 46, No. 3)
   • Observations on GE-Processed Diamonds: A Photographic Record, by Thomas M. Moses et al. (Gems & Gemology, Fall 1999, Vol. 35, No. 3)