Bismuth Glass-Filled Burmese Star Ruby
The Hong Kong laboratory recently examined a 5.35 ct purplish red oval cabochon displaying asterism (figure 1). Standard gemological tests yielded a spot refractive index (RI) of 1.76, medium red fluorescence under long-wave ultraviolet (UV) light, weak red fluorescence under short-wave UV, and a diagnostic spectrum in the handheld spectroscope, all of which were consistent with ruby.
Magnification revealed iridescent silk and arrowhead-like inclusions similar to those frequently found in Burmese rubies. Apart from the natural inclusions, easily observable foreign substances for clarity enhancement were found along surface-reaching fractures and cavities. Numerous rounded to flattened gas bubbles and blue flashes were visible within filled fractures (figure 2, left). The filler also exhibited a different luster from that of the ruby under reflected light. X-ray radiography indicated heavy element depositions along these fractures and cavities (figure 2, right). The Fourier-transform infrared (FTIR) spectrum showed two broad absorptions centered at 3500 and 2670 cm–1 associated with manufactured glass. Qualitative analysis using energy-dispersive X-ray fluorescence (EDXRF) spectroscopy revealed the presence of bismuth, whereas no lead was detected. This confirmed that the foreign material was actually a bismuth-based glass.
Based on internal features and advanced testing results, the stone was positively identified as a manufactured product consisting of bismuth glass and ruby. Although bismuth-based glass has occasionally been applied to corundum as a filling material (Spring 2017 Lab Notes, p. 94), it is rare to detect it in star ruby.
