Article: Beware - Lead-Glass Filling in Colour-Change Sapphire

From Gems & Gemology: Lead-Glass Filling in Colour-Change Sapphire

Gagan Choudhary of the Gem Testing Laboratory in Jaipur, India, submitted the following report to Gems & Gemology's (G&G) Gem News International section:

Lead-glass-filled rubies were first seen in the market in 2004. Much research has since been conducted to classify these rubies correctly, and the treatment is now well understood. We also know, though, that such fillings are now being applied to sapphires...
... as well.

The Gem Testing Laboratory in Jaipur recently encountered a 3.76-ct. oval mixed cut sapphire with numerous eye-visible fissures. The refractive index and specific gravity values were consistent with corundum. The stone appeared brownish-green in daylight and fluorescent light (top image) and brownish-purple in incandescent light (bottom image). It fluoresced weak reddish-orange under long-wave UV radiation; it was inert to short-wave UV. Observation with a desk-model spectroscope showed a doublet in the red region (typically associated with chromium in corundum), a weak absorption in the orange-yellow region, and a strong band in the blue region at ~450 nm, probably due to iron.

With magnification, the surface-reaching cracks showed an obvious blue-to-violet flash effect, which changed to greenish-blue as the stone was moved. The fractures also contained large rounded and highly reflective flattened gas bubbles. In addition, white crystallites were observed in the filled cavities. These features are consistent with those reported in lead-glass-filled rubies (e.g. S. F. McClure et al., "Identification and durability of lead-glass-filled rubies," Spring 2006 Gems & Gemology, pp. 22-34).

The presence of a lead-glass filling in this sapphire was further confirmed by energy-dispersive X-ray fluorescence analysis, which revealed a strong lead peak. Other trace elements detected were chromium, iron and gallium, as expected in a color-change sapphire of natural origin.

This is the first lead-glass-filled sapphire seen in our laboratory, and we would not be surprised to see the treatment applied to other sapphire colors in the future.

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(Source: GIA Insider)