by gagan choudhary

A colour-zoned topaz

Author: Gagan Choudhary

(This article was first appeared in Gems & Gemology, Vol 47, No. 3, pp 250-251)

This report is based on a transparent yellowish to greenish brown specimen received for identification at the Gem Testing Laboratory, Jaipur that was striking for its unusual colour zoning. The oval mixed-cut weighed 27.38 ct and measured 21.18 x 16.39 x 9.64 mm. The specimen initially appeared to be a sapphire because of the pattern of its colour zoning, but the lustre and brilliance were lower than would be expected.

Figure 1: This 27.38 ct topaz shows unusual for colour zoning, with a pattern resembling that seen in sapphire.

The stone had a refractive index of 1.621–1.629, with a birefringence of 0.008 and a biaxial positive optic sign. The hydrostatic SG was 3.57, and moderate greenish brown to brown pleochroism was observed. The sample was inert to long-wave ultraviolet radiation and displayed a weak greenish reaction to short-wave UV. No absorption features were observed with the desk-model spectroscope. RI and SG values were consistent with topaz. The RI values indicated an intermediate OH/F composition, while the SG was consistent with F-rich topaz (R. Webster, Gems, 5th ed., rev. by P. G. Read, Butterworth-Heinemann, Oxford, UK, 1994, pp. 150–163).

When the topaz was immersed in bromoform, the colour zones became sharper and more prominent (figure 2), and they were still reminiscent of those seen in sapphire. The angles formed by the zones varied with viewing direction. Viewed from the table, the angles were approximately 40/140° (figure 2, left); from the side, they were 70/110° (figure 2, right). These angles are different from those seen in corundum (60/120°). No reference in the literature to such angles formed by the crystal faces of topaz was found; however, the pyramidal faces {111} and {11̅1} form an angle of 39° (E. S. Dana, Mineralogy, 4th ed., rev. by W. E. Ford, Wiley Eastern Ltd., New Delhi, 1949, pp. 613–615). The intersection of the colour zones did not form a smooth plane but rather a zigzag pattern (again, see figure 2, left). Therefore, the exact orientation of these colour zones is unknown and can only be assumed to be aligned with the pyramidal faces.

Figure 2: The angles formed by the colour zones in the topaz were different when viewed from the table (left) and the side (right). Also note the plane of intersection, which form a zigzag pattern.

With magnification, the sample was mostly clean except for a few tiny transparent crystals associated with streams of fine pinpoints that formed comet-like patterns (figure 3). Similar inclusions were documented in topaz by E. J. Gübelin and J. I. Koivula (Photoatlas of Inclusions in Gemstones, Vol. 2, Opinio Publishers, Basel, Switzerland, 2005, pp. 751–752).

Figure 3: The topaz contained tiny transparent crystal inclusions associated with streams of fine pinpoints, forming “comet-tail” patterns. Magnified 45x

This was the first time when this author encountered topaz with a distinct colour zoning pattern. Later, in the laboratory, few more similar specimens of topaz were seen, reportedly from the state of Orissa in Eastern India.

All photographs and photomicrographs by Gagan Choudhary