corundum, treatments.6, gems

A natural sapphire of unusual color came into the New York lab for identification. The brilliant orange-yellow seemed unnatural, and the lack of fluorescence and absence of an iron line in the absorption spectrum strongly suggested a heat-treated stone. The clincher, however, was an unpolished burned area near the multiplane girdle (figure 13). Figure 14 shows yet another example of incomplete repolishing of a heat-treated orangy yellow natural sapphire. It came from a lot of six stones, all of which clearly owed their color to heat treatment. One yellow sapphire from another lot of four stones recently seen in New York faded appreciably after exposure to south daylight for a day or so. Normally, we would assume that the color was due to irradiation and not to heat, but we recently heard that certain heat-treated yellow natural sapphires may fade (Gems & Gemology, Spring 1982, pp. 47–48). This has not yet been confirmed.