corundum, synthetic corundum.7, treatments.6, gems

The blue oval cut shown in figure 11, which was submitted to the New York lab for identification, is interesting for a number of reasons. The stone has an extremely deep pavilion and a shallow crown, with a total depth-to-width ratio of approximately 115%—so that it is actually deeper than it is wide. In addition, to the unaided eye it appears to be free of inclusions and has a pleasant light blue color. These facts suggested that the stone might be an old, “native cut,” unheated Ceylon sapphire. However, careful examination with the microscope revealed a shadow of curved color banding near the culet and suspicious stress cracks on some facets. The strong chalky blue fluorescence to short-wave UV radiation suggests that the stone either was subjected to extreme heat in treatment or is of synthetic origin. The curved color banding together with the Plato test positively identified it as a Verneuil flame-fusion synthetic sapphire. The stone is also an excellent example of how a very small color zone can produce a pleasant face-up color. Notice in the side view (figure 12) that the top of the stone is colorless; the small blue area near the culet, visible at this angle, is virtually the entire extent of color. Blue synthetic sapphire boules often have color only in a thin layer near the surface. Figure 12 also shows the extreme depth-to-width ratio of this stone.