Gems & Gemology, the quaterly scientific journal of the Gemological Institute of America in its Spring 2021 issue reported the surprising news of the detection of a lab-grown Ruby being filled with Lead glass, a process usually applied to natural rubies using a filling material with a Refractive Index very close to natural rubies, to minimize the appearance of fractures and improve the clarity of very low-qualiity rubies. The biggest question was why someone would resort to treating a synthetic ruby with lead glass filling, a process reserved for natural stones, unless of course the synthetic material too was of very poor quality with multiple fractures.
The GIA Lab in Carlsbad received this ruby for identification, and reports the lab usualy receives glass-filled natural rubies for identification and this is the second instance they had received a glass-filled lab-grown ruby for identification. The GiA further states that while it is unclear, why anyone would go to the trouble to treat a synthetic ruby with lead-glass filling, gemologists should be aware that such material does exist in the trade.
Initial investigations on the transparent to semi-transparent, oval mixed-cut specimen weighing 3.53 carats showed that it possessed all the gemological properties of natural rubies, such as a refractive index falling within 1.761 and 1.769, a birefringence of 0.008, a specfic gravity of 4.01, a uniaxial optical figure and a characteristic spectrium of rubies as seen under a handheld spectroscope.
Evidence for lead-glass filling of the fractures in the stone was provided by its examination under magnification, which showed a network of interconnected fractures with dendritic patterns and coarse flattened gas bubbles and a pronounced blue and orange flash effect throughout, which indicated that the fractures were filled with lead glass.
Since, no inclusions were observed to indicate the natural or synthetic origin of the corundum, one has to resort to fluorescent studies to establish its origin. The stone exhibits strong red fluorescence to long-wave ultraviolet (LWUV) and medium chalky bluish-white fluorescence to short-wave ultraviolet (SWUV). Chalky SWUV is typical of heated ruby and sapphire, but for synthetic sapphire and ruby it makes it more difficult to observe diagnostic curved striae in the stone. In this sample it was much harder, since that effect was combined with iinterference from filled fractures.
A stronger SWUV light source would be more effective in such cases, and curved growth lines were observed in the DiamondView as chalky luminiscent bands, a convincing proof of the laboratory-grown origin of the stone.
Lead-glass filling in corundum is easily detected with magnification, the identifying characteristics being very low-relief fractures, flattened and rouinded gas bubbles, unfilled areas or voids in fractures and a blue and orange flash effect. This ruby displayed an orange and blue flash effect as seen earlier, as well as gas bubbles trapped in fractures that provided conclusive evidence for lead-glass filling of the fractures in this symthetic ruby.