Heat treatment

Controlled heating of a stone to improve its colour or clarity, most commonly seen in corundum, where it dissolves silk and develops colour. The result is stable and permanent, and the treatment is legitimate when disclosed. 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. Distinct from diffusion, in which an element is added during heating and which must be disclosed separately.