Unheated sapphire

Unheated (or "no heat") describes a sapphire that has never been thermally treated to improve color, clarity, or transparency. The disclosure is required on every SSEF, Gübelin, AGL, and GRS report; "no heat" or "no indications of heat treatment" is the standard language. Roughly 5% of commercial sapphire entering the trade is unheated; the remaining 95% has been heated at some point in the supply chain.

The trade premium for unheated sapphire is meaningful and tier-dependent. At investment-grade quality (3 ct+ Kashmir, Mogok, top-quality Ceylon and Madagascar with origin paper) unheated stones trade at 3x to 10x heated equivalents at the same visible color. At commercial grades (sub-3-ct Ceylon, Madagascar, Australia, Thailand) the premium is smaller — typically 30% to 80%. At industrial-grade material the premium is negligible because heat is the industry default.

The technical case for unheated is that the color is "natural" — formed during the geological cooling history of the crystal rather than introduced by a 1,400-to-1,800-degree industrial process. The visual case is that unheated stones retain silk inclusions, which produce the velvety character of cornflower blue Kashmir and the characteristic light play of fine Mogok rubies. Heated stones look brighter and more saturated but lose the silk and the velvet.

The determination is made by SSEF, Gübelin, AGL, or GRS via microscopic inspection (silk presence, undisturbed rutile needles, undisturbed inclusion fingerprints) and FTIR spectroscopy (heated stones show characteristic absorption peaks at OH-related wavelengths that unheated stones do not). The "no heat" call is reliable when issued by these labs; lower-tier labs sometimes issue "no heat" on stones that have been low-temperature heated below silk-dissolution temperature, which the top-tier labs would catch.

Heat treatment is reversible at the visible-evidence level — the silk does not regrow after destruction, so once destroyed the "no heat" determination becomes impossible. A stone heated above silk-dissolution will always read as heated. Low-temperature heating below silk-dissolution (around 1,000 to 1,200 degrees) is sometimes used to refine color without destroying silk; SSEF and Gübelin catch this via FTIR even when silk is intact. The "no heat" claim requires both intact silk and clean FTIR.

The buyer's rule: at 3 ct and above on a Kashmir, Mogok, or top-quality Ceylon sapphire, "no heat" with SSEF or Gübelin paper is the credential the market prices. Below 3 ct the premium is smaller; below 1 ct the premium is often not worth the difference in availability.