corundum, synthetic corundum.7, gems

The West Coast laboratory recently received for identification the synthetic brown star sapphire ring illustrated in figure 14. Although synthetic star sapphires of this color are known (the GIA collection has some samples that were produced by Linde more than 20 years ago; similar material is also illustrated in K. Nassau’s Gems Made by Man, Chilton Book Co., Radnor, PA, 1980), this is the first one that we recall having received at this laboratory for identification. Although this stone is basically opaque, it has some semitranslucent areas near the surface. The stone has an R.I. of 1.76 (determined by the spot method), is inert to both long- and short-wave ultraviolet radiation, and shows no absorption lines or bands in the visible spectrum (400–700 nm). When the stone was viewed with the unaided eye, widely spaced curved color bands were visible, as well as the sharp star. These bands, which are medium brown and dark yellow-brown, could be followed around the top of the cabochon in a concentric pattern; they were only faintly visible on the base of the stone. With magnification, we observed tiny spherical gas bubbles just under the surface, as well as irregular veins and dark brown shallow cracks.