Lead Glass–Filled Laboratory-Grown Ruby
The GIA lab in Carlsbad received for identification a lead glass–filled laboratory-grown ruby (figure 1). We commonly see glass-filled natural rubies, but to our knowledge only one other laboratory-grown example has ever been submitted.
This transparent to semi-transparent oval mixed cut weighed 3.53 ct and showed gemological properties of ruby: a refractive index of 1.761–1.769, a birefringence of 0.008, a specific gravity of 4.01, a uniaxial optic figure, and a characteristic ruby spectrum using a handheld spectroscope.
Examination of the stone under magnification showed a network of interconnected fractures. A pronounced blue and orange flash effect was observed throughout, proving the fractures were filled with lead glass. Also present in the fractures were dendritic patterns and coarse flattened gas bubbles (figure 2).
No inclusions were observed to indicate the natural or synthetic origin of the corundum. However, this stone exhibited strong red fluorescence to long-wave ultraviolet and medium chalky (bluish white) red short-wave ultraviolet fluorescence. Chalky SWUV is typical of heated ruby and sapphire. But when viewed in synthetic pink sapphire and synthetic ruby, it generally makes it more difficult to observe diagnostic curved striae in the stone. In our sample it was much harder, since that effect was combined with interference from filled fractures.
In these cases, a stronger SWUV light source may prove helpful. The curved growth lines were seen as chalky luminescent bands in the DiamondView, proof of its laboratory-grown origin (figure 3).
Lead-glass filling of corundum is based on the same principle applied to emerald and diamond: the use of a filling material with an RI very close to that of the host material, to minimize the appearance of fractures. The treatment is very effective, and the clarity of very low-quality material can be improved, rendering it usable in jewelry. This makes it possible to market a great deal of previously unsalable material. Fortunately, this treatment is easily detected with magnification. The identifying characteristics are very low-relief fractures, flattened and rounded gas bubbles and voids (unfilled areas) in fractures, and a blue and orange flash effect (S.F. McClure et al., “Identification and durability of lead glass–filled rubies,” Spring 2006 G&G, pp. 22–33). This stone displayed an orange and blue flash effect (again, see figure 2) as well as gas bubbles trapped in fractures that were conclusive evidence for lead-glass filling of the fractures in this synthetic ruby. 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.