corundum, sapphire, color.4, inclusions.5, treatments.6, East Africa.12, Sri Lanka.12, Tanzania.12, gems

Investigates the causes of colour and identification of naturally coloured, irradiated and heat-treated yellow to orange-brown sapphires. Natural coloration is attributed to colour centres and/or trace elements, while irradiation and heating can produce comparable hues in colourless or pale yellow corundum. Both treatments generate colour centres associated with increasing absorption towards the blue end of the visible spectrum and a broad band near 465 nm. Irradiation-induced colours are highly unstable, whereas the heat-induced colour centres studied persist on reheating to 1000°C. Absorption spectra distinguish several colour mechanisms, although treated stones can closely resemble naturally pale yellow Sri Lankan sapphires. Heat-altered inclusions provide additional evidence of treatment.