gems, corundum, treatments.6, irradiation, irradiation paper on ruby

The sapphire observations in these laboratory notes address the identification of treatment and the distinction between natural origin, artificial colour modification and colour stability. A colourless natural sapphire from the laboratory collection, stored in a packet identifying it as an irradiated yellow sapphire, illustrated the reversibility of radiation-induced colour. Its former yellow appearance had faded, but a ten-minute exposure to x-rays at 45 kV and 19 mA, close to the tube window, restored the colour. The account explains that some colourless sapphires become yellow readily under x-ray irradiation and subsequently return towards their original colourless or pale yellow state through exposure to sunlight. Consequently, establishing that a sapphire is natural does not establish either that its colour is natural or that the colour will remain unchanged. Tests intended to assess stability may themselves alter a stone’s appearance and therefore require the owner’s permission. A second case concerns two intensely orange sapphires weighing 32.44 and 8.72 ct, whose unusual hue resembled Verneuil synthetic orange sapphire more closely than the heat-treated yellow sapphires commonly examined by the laboratory. Their refractive indices were normal for corundum, while their absorption showed strong attenuation in the blue and violet, without sharp red lines or the usual sapphire bands in the blue. Microscopy revealed numerous dust-like particles responsible for a cloudy appearance. These formed very fine silk, generally aligned in particular directions but also arranged around small angular areas. Other internal features, including glassy centres in some feathers, supported identification as natural sapphires that had undergone heat treatment. The exceptional colour and intensity nevertheless left open the possibility of additional irradiation or unstable colour associated with treatment. Controlled fading tests were proposed, but permission was refused, leaving this question unresolved. Together, the observations demonstrate the complementary roles of refractometry, absorption spectroscopy, microscopy and colour-stability testing, while showing the limits of conclusions that can be drawn when potentially colour-altering examinations cannot be performed.