Lab Notes Gems & Gemology, Spring 2020, Vol. 56, No. 1

Bismuth Glass-Filled Burmese Star Ruby

A 5.35 ct Burmese star ruby.
Figure 1. A 5.35 ct purplish red cabochon displaying asterism. Photo by Johnny Leung.

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.

Detailed views of surface-reaching fractures and cavities.
Figure 2. Left: Blue flashes and trapped gas bubbles of various sizes and shapes along surface-reaching fractures and cavities. Photomicrograph by Xiaodan Jia; field of view 7.64 mm. Right: The bismuth glass-filled fractures and cavities were easily visible in the X-ray image. Image by Sze Ling Wong.

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.

Xiaodan Jia is senior staff gemologist, and Mei Mei Sit is supervisor of colored stones identification, at GIA in Hong Kong.