Sphalerite in Topaz
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.