Gem News International Gems & Gemology, Spring 2014, Vol. 50, No. 1

Yellow Sapphire Filled with Lead Glass

Glass-filled yellow sapphire
Figure 1. This 4.27 ct sapphire was filled with orangy yellow lead glass throughout the stone. Photo by Gagan Choudhary; courtesy of Dheeraj Gupta.
This contributor has previously reported glass-filled, color-changing red and blue corundum (Spring 2008 GNI, pp. 88–89). After recent reports of green sapphire filled with lead glass (Fall 2013 Lab Notes, p. 176) and sapphires with cobalt-colored lead glass (e.g., T. Leelawatanasuk et al., “Cobalt-doped glass-filled sapphire; an update,” Australian Gemmologist, Vol. 25, No. 1, 2013, pp. 14–20), an orangy yellow sapphire filled with lead glass was presented to us by Dheeraj Gupta (Gem & Jewellery Sales Promotion Council, New Delhi).

The 4.27 ct sapphire (figure 1) was hazy throughout, with a roiled effect reminiscent of hessonite, but standard gemological properties and microscopic analysis identified the specimen as natural corundum. It gave an RI of 1.762–1.770; a hydrostatic SG of 4.03; an absorption band at around 450 nm, along with fine lines in the red end under the desk-model spectroscope; and reddish orange fluorescence in long- and short-wave UV. The sapphire showed stronger fluorescence under long-wave UV. Microscopic examination revealed numerous low-relief cracks throughout the stone with a distinct flash effect. Also visible were trapped, flattened gas bubbles, as seen in figure 2, and whitish cloudy patches; these are typically associated with filled stones. Also present were milky zones consisting of fine discs (usually rutile) and negative crystals associated with liquid films. Under diffused lighting, the glass-filled fractures appeared orangy yellow against the pale yellow bodycolor of the stone (again, see figure 2). As a result, the face-up color also appeared orangy yellow. The color of the filler glass was similar to that observed in glass-filled rubies. The lead content of the glass was further confirmed by EDXRF analysis, while the presence of iron (the cause of the sapphire’s yellow color) was confirmed by an Fe-related 450 nm band in the UV-visible spectrum.

4.27 ct sapphire was filled with orangy yellow lead glass
Figure 2. The color of glass-filled fractures appeared orangy yellow against the sapphire’s pale yellow bodycolor; thus, the face-up color also appeared orangy yellow. Note the spherical bubbles trapped within the fractures. Also present were flattened bubbles and a typical flash effect (not shown here). Photomicrograph by Gagan Choudhary, magnified 32×.
Since this was first example of lead-glass-filled yellow sapphire examined by this contributor, its market penetration is still unknown. With its transparency and color, this material qualifies as an inexpensive substitute for yellow sapphire, provided there is complete disclosure. 

Gagan Choudhary is deputy director of the Gem Testing Laboratory in Jaipur, India.