Heating corundum at high temperature can improve colour and clarity by dissolving silk and shifting the chromophores that make a sapphire blue or yellow. It is permanent and stable, and it has been standard practice for decades. Most sapphires on the market have been heated.
The listing values
Every listing uses one of six treatment values: UNHEATED, HEATED, DIFFUSION_TREATED, FRACTURE_FILLED, OTHER or UNKNOWN. HEATED covers conventional heating without additives. DIFFUSION_TREATED covers beryllium or titanium diffusion, which introduces colour from the surface inward and must be disclosed. FRACTURE_FILLED covers lead-glass or resin filling, which affects durability. OTHER is used with a free-text explanation, and UNKNOWN means the supplier did not state a treatment.
Why unheated is different
Unheated is a negative claim: no indications of heating. It cannot be established by looking at the stone with a loupe, so the trade relies on laboratory testing of inclusions and spectra. On this platform, the value UNHEATED is displayed with a supplier-reported label unless a laboratory report is attached to the listing, in which case it is labelled lab documented and the report link is shown. Always read the report itself; wording varies between laboratories.
Last updated 2026-09-22.