gems, corundum, treatments.6, Mozambique.12

Gemologists at Bangkok’s Lotus Gemology recently received a 6 ct ruby for identification. Declared to be an untreated ruby from Mozambique, the stone featured a superb vivid red color of the type often termed “pigeon’s blood” in the trade. As it was also of good clarity and well cut, it was clear we were dealing with a gem of potentially high value. The UV-Vis-NIR spectrum was typical for ruby or synthetic ruby. The infrared spectrum revealed peaks at 3309 and 3232 cm⁻¹. The 3232 cm⁻¹ peak generally indicates that a stone has been subjected to artificial heat treatment. We proceeded to examine the gem under the microscope, where we found two additional pieces of evidence of heat treatment. The first feature was rutile silk typical of rubies from East Africa. In these stones, the silk consists of high-relief needles of rutile with attached unidentified “daughter” crystals of another substance of lower relief. In heat-treated stones, the daughter crystals will sometimes show partial breakdown (“GIA Lab Reports on Low-Temperature Heat Treatment of Mozambique Ruby,” GIA Research News, April 28, 2015). Small areas of breakdown in the form of irregular white patches were found in this stone (figure 1). But the most obvious evidence of heat treatment was the presence of spall marks (figure 2), which are solidified droplets of material on the surface that melted or were dissolved during heating. These spall marks across the stone showed that the ruby had been heated after cutting and had not been repolished following treatment. We can only guess at the reasons for heating such a valuable gem, but there is no doubt that this particular roll of the dice was a losing gamble because the potential improvement in appearance was slight compared to the large price difference between untreated and heated ruby. We concluded that this was a heat-treated Mozambique ruby.