corundum, synthetic corundum.7, flux, Verneuil, gems

A group of orange synthetic corundum crystals supplied by Tom Chatham is described, with particular attention to crystal habit, inclusions, luminescence and colour stability. The specimen weighs slightly more than 13 ct and consists of several crystals reaching approximately 1 cm in height. They display a basic rhombohedral form and are less bladed than blue synthetic sapphire crystals previously produced by the same firm. Their attractive reddish-orange colour shows some zoning, although this is less pronounced than in the blue material. The crystals are thick enough for faceting, but numerous inclusions are present, particularly angular flakes large enough to be seen without magnification. These are compared with inclusions identified as platinum in an earlier study by R.E. Kane. A substance containing numerous gas bubbles occurs on the back of the group. Drawing on Kane’s account and information from the manufacturer, O’Donoghue explains that such material is a silica-based ceramic glaze applied to crystal groups and fired at 1000 °C, a practice reported for both orange and blue Chatham groups. The most striking observations concern the response to filtered illumination and the spectrum. When illuminated by a high-intensity lamp through a copper sulphate solution and viewed through a red gelatine filter, the specimen glows strongly reddish orange. A similar but weaker response occurs under long-wave ultraviolet radiation. Examination with a hand spectroscope reveals a conspicuous sharp emission line in the deep red, readily visible even to inexperienced observers. No colour change was observed after several days in daylight or after ultraviolet exposure for one hour on each of two occasions. The author distinguishes these observations from previously reported X-ray-induced colour changes in some blue Chatham sapphires. Strong pink-orange and brownish-yellow pleochroism broadly agrees with Kane’s description. Specific gravity and refractive indices were not measured on this specimen; values quoted from Kane’s separate specimen are approximately 4.00 and 1.762–1.770, with birefringence of 0.008. The note supplements the earlier investigation through observations on another crystal group while distinguishing directly observed properties from previously published measurements.