Heat treatment

Heat treatment is the controlled heating of a stone to improve its colour or clarity, most commonly seen in corundum, where a furnace run above a thousand degrees dissolves silk, clears milky haze and develops colour: pale Sri Lankan geuda material turns commercial blue, brownish ruby turns red, yellow sapphire gains saturation. Aquamarine, tanzanite and much tourmaline are routinely heated too, and the trade barely mentions it. The result is stable and permanent, and the treatment is legitimate when disclosed. Most sapphire and ruby on the market has been through a furnace.

Heated and unheated stones of equal beauty trade at different prices, which is why the treatment line on a report matters more than any adjective in a listing. The gap widens with size and quality. At commercial grades it is modest; for fine colour in a stone of a few carats it is the largest single factor in the price after origin.

On a lab report

Each house has its own phrasing, and it pays to know it. SSEF and Gübelin write "no indications of heating" for an unheated stone and "indications of heating" for a heated one. GIA uses the same two phrases. GRS uses a lettered code, H for heated, with a bracketed grade where heating has left residues in fissures, and writes "no indication of thermal treatment" for the unheated case. AIGS and Lotus follow the same logic in slightly different words. Distinct from all of this is diffusion, in which an element such as beryllium or titanium is added during heating; it must be disclosed separately, and the report names it in the treatment line alongside the heating.

Ask "heated or unheated" and get the answer in writing on the invoice, backed by a current report whose number checks online. A dealer's "probably no heat" is not a treatment statement. It is a guess with your money behind it.