Glass-Filled Yellow Sapphire - an overview
by Gagan Choudhary
Due to the extensive popularity of glass-filled rubies, experiments have been conducted on various other colours of corundum too, but they could not get so much popularity. However, glass-filling in blue sapphires using cobalt, imparting blue colour to the otherwise white or colourless corundum, introduced in the year 2012 (e.g. Leelawatanasuk et al, 2013), has become very popular. Complete disclosure although remained a concern. While there are reports of glass-filling on other colours of corundum, such as colour changing (Choudhary, 2008) and green (York, 2013), an orangy yellow sapphire (4.27 ct) filled with lead-glass was presented to him by Dheeraj Gupta of Gem & Jewellery Sales Promotion Centre, New Delhi.
Figure 1: This 4.27 ct sapphire was filled with orangy yellow lead glass
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. The specimen 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 desk model spectroscope; and reddish orange fluorescence in long- and short-wave UV. The sapphire showed stronger fluorescence under long-wave than short-wave. The SG value was slightly on the higher side but within the limits of corundum.
Microscopic examination is one of the most reliable methods for identification of glass-filling in corundum. It revealed numerous low-relief cracks throughout the stone with a distinct flash effect, associated with a filler of similar refractive index to that of the host (figure 2); also visible were trapped, flattened gas bubbles, as seen in figures 3 and 4, and whitish cloudy patches. These are typically associated with filled stones. Under diffused lighting, the glass-filled fractures appeared orangy yellow against the pale yellow bodycolour of the stone (again, see figures 3 and 4). As a result, the face-up colour also appeared orangy yellow. The colour of the filler glass was similar to that observed in glass-filled rubies. Also present were milky zones consisting of fine discs (usually rutile) and negative crystals associated with liquid films (figure 5).
Figure 2: The lead-glass filler produced dominantly green and blue colour flashes. Magnified 32x
Figure 3: The colour of glass-filled fractures appeared orangy yellow against the sapphire’s pale yellow bodycolour; thus, the face-up colour also appeared orangy yellow. Note the spherical bubbles trapped within the fractures. Magnified 32×