Translucent Ruby Filled with Zinc Glass
Recently the Carlsbad laboratory received an 8.57 ct translucent purplish red heart-shaped mixed-cut stone for identification service (figure 1). Standard gemological testing revealed a refractive index of 1.760–1.769, and a ruby spectrum was obtained with a handheld spectroscope. The stone showed a medium to strong red fluorescence to long-wave UV radiation and a very weak red fluorescence to short-wave UV radiation. During microscopic analysis, several fractures containing a whitish filler and air pockets were seen using fiber-optic lighting (figure 2). Using reflected light, fractures with a lower luster than the host corundum were also observed, which confirmed the presence of a filling material.
The stone did not show the blue flash effect typically observed in rubies filled with lead glass (S.F. McClure et al., “Identification and durability of lead glass–filled rubies,” Spring 2006 G&G, pp. 22–36). Although the filler improved the durability, it did not appear to improve the clarity.
Glass filling treatment has been used on heavily fractured rubies to increase their clarity and durability since the early 1980s, and cavity filling was noted and described as early as 1984. The original filler was silica glass, which was easily visible since its refractive index (~1.5) is significantly lower than corundum (R.E. Kane, “Natural rubies with glass-filled cavities,” Winter 1984 G&G, pp. 187–199). In early 2004, gemologists discovered a new type of glass filler with high lead content and a much better clarity enhancement due to its higher reflective index (~1.70) (GAAJ Research Laboratory, “Lead-glass impregnated ruby,” March 15, 2004). Since then, lead glass fillers have become the most popular filler for rubies, although other glass types such as bismuth and cobalt have also been used (T. Leelawatanasuk et al., “Cobalt-doped glass-filled sapphire: An update,” Australian Gemmologist, Vol. 25, No. 1, 2013, pp. 14–20; Spring 2020 Lab Notes, p. 139).
In the heart-shaped ruby submitted to the laboratory, energy-dispersive X-ray fluorescence spectroscopy did not detect lead or bismuth but did show chromium, iron, and zinc. Additional chemical analysis was performed with laser ablation–inductively coupled plasma–mass spectrometry on two spots of one larger fracture with a substantial amount of filler in order to quantitatively measure the elements in the glass filler. The averages of the two analysis spots were 28750 ppmw silicon, 1091 ppmw zinc, and 108 ppmw lead.
Chemical results and gemological properties revealed that this glass filler was not the lead or bismuth glass the authors first suspected, but rather a silica-based glass doped with zinc. While this type of filler had visual properties similar to those of other glass fillers in corundum, it did not show a flash effect. This is the first time the authors have encountered a ruby filled with zinc glass.
