Gem News International Gems & Gemology, Summer 2014, Vol. 50, No. 2

Unusual Optical Effect in Blue Sapphire

Iurii Gaievskyi, Igor Iemelianov, and Elena Belichenko

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Sapphire with color shift
Figure 1. This 1.36 ct light bluish violet sapphire displayed an interesting color shift when exposed to the UV radiation of the DiamondView. Photo by Igor Iemelianov; magnified 15×.
A 1.36 ct light bluish violet sapphire (figure 1) was presented to the laboratory of the State Gemological Center of Ukraine for examination. The stone exhibited an RI of 1.762–1.770, with a birefringence of 0.008, and a hydrostatic SG of 3.97. The samples displayed typical pleochroism and a very weak pinkish orange fluorescence to long-wave UV. Qualitative analysis using EDXRF spectroscopy showed major amounts of Fe, Cr, and Mg, and minor amounts of Ti and Ga. Microscopic examination revealed dot-like dissolved rutile, proof of heat treatment. The sapphire showed a small color shift from bluish violet in the daylight to pale blue in incandescent light. A band with pink luminescence (apparently due to Cr3+ impurity) was identified using the DiamondView (figure 2). When removed from the DiamondView, the stone had completely changed color to brown (figure 3, left). After about 15 minutes in natural daylight, the light bluish violet hue began to return (figure 3, right). About 12 hours later, its original blue-violet color was restored. Such color instability is extremely unusual in blue sapphires.

Sapphire with luminescent band
Figure 2. The DiamondView recorded this band with pink luminescence in the light bluish violet sapphire. Image by Igor Iemelianov.
Sapphire reverts to original color
Figure 3. When removed from the DiamondView, the sapphire had completely turned brown (left). After about 12 hours, the stone reverted to its original light bluish violet color (right). Photos by Igor Iemelianov, magnified 15×.
It should be noted that the UV lamp in the DiamondView has a wavelength less than 225 nm (according to the user’s manual). The energy of UV radiation is considered too weak for the color treatment of corundum (see K. Nassau, Gemstone Enhancement, Butterworth-Heinemann, 1984, p. 221). In this sapphire, the DiamondView apparently caused the color change. Such a reversible change of color with exposure to light is called photochromism.

Iurii Gaievskyi (gaevsky@hotmail.com), Igor Iemelianov, and Elena Belichenko are with the State Gemological Centre of Ukraine, Kiev.