Gem News International Gems & Gemology, Summer 2022, Vol. 58, No. 2

Ruby: An Expensive Mistake

Heat caused irregular white patches to form due to the breakdown of lower-relief crystals.
Figure 1. Silk in corundum often unmixes into two different solid phases, one highly reflective (rutile) and the other of lower relief (ilmenite or hematite-ilmenite). When the Mozambique ruby was heated, the lower-relief crystals began to break down, developing irregular white patches, as shown in the yellow circle. Photomicrograph by Richard W. Hughes; field of view 1 mm.

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 a type that is often termed “pigeon’s blood” in the trade. As it was also of good clarity and well cut, it was obvious we were dealing with a gem of potentially high value.

The UV-Vis-NIR spectrum was typical for ruby/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 in the microscope, where we found two additional pieces of evidence of heat treatment.

The first feature was rutile silk of a type that is 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 the case of heat-treated stones, the daughter crystals will sometimes show a partial breakdown (“GIA Lab reports on low-temperature heat treatment of Mozambique ruby,” GIA Research News, April 28, 2015). Small amounts of breakdown in the form of irregular white patches were found in this stone (figure 1).

Spall marks on ruby’s surface are evidence of heat treatment.
Figure 2. Spall marks on the surface of the Mozambique ruby prove it was heat treated after cutting and polishing. Note that some of the spall marks have dark halos around them. Photomicrographs by Richard W. Hughes; fields of view 2.0 and 2.5 mm.

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 not repolished following the treatment. We can only guess about 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 is slight compared to the large price difference between untreated and heated ruby. We concluded that this was a heat-treated Mozambique ruby.

Richard W. Hughes is an award-winning gemologist and co-founder of Lotus Gemology in Bangkok.