Micro-World Gems & Gemology, Fall 2016, Vol. 52, No. 3

Sphalerite in Topaz

Sphalerite crystal inclusions in topaz
Figure 1. Oblique diffuse illumination reveals the scene within a colorless topaz. Its sphalerite inclusions all feature an isometric crystal habit, metallic luster reflections, and subtle iridescent interface colors. Photomicrograph by Jonathan Muyal; field of view 4.79 mm.

A 6.54 ct colorless topaz specimen hosting eye-visible dark crystals was examined at GIA’s Carlsbad laboratory. The crystals exhibited a submetallic luster and showed isometric morphology with step-like growth (figure 1). Some also displayed slightly corroded rounded edges with a reflective iridescent interface. The corroded appearance suggested that these dark crystals are protogenetic inclusions that were present in the growth environment before the topaz began to form. Raman spectrometry and LA-ICP-MS analysis confirmed that the inclusions were sphalerite, a zinc-iron sulfide with the formula (Zn,Fe)S. Fluid inclusions were also observed in this topaz.
Sphalerite has been described in the literature as a collector’s gem for its rarity and beautiful luster (Summer 1992 Gem News, p. 202). This is the first documented example of sphalerite as a crystal inclusion in colorless topaz, making it an unusual collector’s gemstone.

Jonathan Muyal and Ziyin Sun are staff gemologists at GIA’s laboratory in Carlsbad, California. Nathan Renfro is analytical manager of the gem identification department and microscopist of the inclusion research department, also at GIA’s Carlsbad lab. Aaron Palke is a lecturer in mineralogy at the University of Queensland and senior curator in mineralogy at Queensland Museum in Brisbane, Australia.