Why does it matter when a seller says a stone is diffusion treated? Because its colour has been artificially changed, and the change may be only skin-deep. Lattice diffusion uses heat and chemicals to move an element from outside into a gemstone and change its colour 1. Gemmologists once called it bulk diffusion or surface diffusion, but lattice diffusion is the scientifically correct name 4. Unlike heat treatment alone, it adds colour-causing chemicals during heating 3. Diffusion-treated sapphire is usually less valuable than heated sapphire 2.
Which stones get it, and does the colour last? It is used on corundum, the mineral that forms sapphire and ruby. With beryllium, it has produced yellow, blue, ruby and orange-pink colours 4. The added colour is sometimes shallow and sometimes goes all the way through 1. A thin surface layer survives routine cleaning, including ultrasonic and steam. Repolishing or recutting, though, can remove part or all of it 3.
How is it found and declared? A gemmologist can place the stone in a liquid and look for colour gathered along facet edges and around the girdle (the stone’s outer edge) 3. Some beryllium-treated stones need costly lab chemical analysis to identify 4. GIA identification reports state any treatments that can be detected 2. Under CIBJO guidelines, sellers must disclose it as "diffusion treated" before the sale, next to the stone’s name on the invoice 6. See how gemstone treatments should be disclosed.
Example: A seller offers a bright orange-pink sapphire labelled "diffusion treated". That label tells you its colour was artificially changed. Recutting could remove some of that colour, so you may want a lab report before you decide.