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These two rubies contain numerous fractures filled with a high-lead-content glass that undoubtedly improved their apparent clarity. Photo by Maha Tannous.
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Heat treatment of natural rubies frequently leaves a glassy residue within surface-reaching fractures and cavities. It is often detectable with magnification by observing the difference in luster compared to the surrounding corundum. This type of glass residue, which may be a component of a flux-facilitated healed fracture, has a relatively low refractive index (RI) compared to corundum and is usually not considered a clarity enhancement on its own.
Earlier this year, however, the Gemmological Association of All Japan (GAAJ) issued a Lab Alert describing rubies that had not been heat treated, but that showed a flash effect in their fractures caused by a clarity enhancement similar to that traditionally used in diamonds. Their energy-dispersive X-ray fluorescence (EDXRF) chemical analyses revealed the presence of elevated levels of lead (Pb) in the material filling the fractures in these stones. The results were later confirmed by the American Gem Trade Association.
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| The glassy fillings in these clarity-enhanced rubies all contained numerous flattened gas bubbles. Photomicrograph by Shane F. McClure; magnified 27×. Photo by Maha Tannous. |
Recently, GIA's West Coast laboratory had the opportunity to examine two purplish red mixed-cut oval stones (3.19 and 2.76 ct.) that contained numerous large surface-reaching fractures and cavities filled with a glassy substance (figure 1). Both gems were identified as natural ruby by their RI’s, inclusions, and visible absorption spectra in the desk-model spectroscope. Unlike the stones described by GAAJ, the presence of thermally altered inclusions showed that these rubies had been heated. The glassy fillings contained numerous flattened gas bubbles or voids (figure 2), and showed a distinctly higher surface luster than the corundum (figure 3). The filled fractures were very low relief in all viewing directions and displayed weak-to-moderate blue-to-violet and orange flash effects. In some directions, the fractures had a slightly hazy appearance. The 3.19-ct. ruby, in particular, had several large filled cavities.
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| The luster of this filler material was actually higher than the ruby, which is just the opposite of the situation typically seen with glassy residues. Surface cavities filled with this lead glass also showed a poor polish compared to the surrounding corundum, indicating a much lower hardness. Photomicrograph by Shane F. McClure; magnified 40×. |
The filling material in the largest cavity was translucent, yellow in color, and contained many spherical gas bubbles. The filling material in the fractures did not have a visible body color, but this was undoubtedly because of the small amount of material present. All the filled cavities showed poor polish at the surface, indicating a much lower hardness than the surrounding corundum (again, see figure 3). The nature and abundance of the fillings in these two stones suggested an intent to hide the fractures and improve the apparent clarity of the gems.
To learn more about the filler material, we employed EDXRF analysis, Raman spectroscopy, and fluorescence imaging. EDXRF, performed by GIA senior research associate Sam Muhlmeister, showed elements typical of ruby (Al, Cr, and Fe) and elevated levels of Pb similar to those reported by GAAJ. None of the other elements that have been reported in glass-like ruby fillings (Si, P, Ca, and Ti) were detected. It is important to note that lighter elements such as boron cannot be measured using EDXRF.
Raman spectroscopy was performed to compare the glass filler in these two rubies with five different glass samples known to contain significant Pb. Although the spectrum for this filler was not a match for any of the glasses in our collection, it had many of the same luminescence features as a sample of lead borate glass.
Last, we observed reactions to high-energy short-wave UV radiation using the De Beers DiamondView instrument. The filling material responded very strongly, fluorescing bright blue in contrast to the red reaction (caused by chromium) of the surrounding corundum. Traditional glass-filled cavities in rubies show an inert to dull gray reaction.
Key gemological identification features of this new filler include a distinct luster, flash effects, haziness, gas bubbles and voids, together with very low relief of the fractures in all viewing directions. In addition, EDXRF analyses of the samples we have seen thus far showed elevated Pb but none of the other significant elements attributable to a filler. Blue fluorescence in the DiamondView may also be useful for identification.
Kimberly M. Rockwell and Christopher M. Breeding
GIA Gem Laboratory
Carlsbad, California
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