When trying to identify Thai bulk diffusion of sapphire, there are two indicators to look for: evidence of high heat and evidence of a surface conformal color layer.

When looking for evidence of high heat, “Look for an inclusion scene that suggests extreme conditions, where inclusions have been strongly altered,” says Tom Moses of the Gemological Institute of America’s Gem Trade Laboratory (GTL).

To detect evidence of a surface conformal color layer, look from the surface into the stone for a color layer that looks like the outside surface of the polished stone. “In some cases, the color zoning is really quite deep, but it is still there,” says the American Gem Trade Association’s Ken Scarratt.