Gem News International Gems & Gemology, Winter 2013, Vol. 49, No. 4

Faceted Wurtzite from Tanzania

Jaroslav Hyršl

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Figure 1. These wurtzites are from Merelani, Tanzania. The crystal measures 17 mm across, and the cut stones weigh 1.74 and 1.59 ct. Photo by Jaroslav Hyršl.
Wurtzite is the hexagonal polymorph of ZnS. It is far less common than sphalerite, the cubic polymorph of ZnS. All previously identified wurtzite has been opaque to translucent, and therefore not facetable. Layers of wurtzite and sphalerite can alternate in so-called Schalenblende, a colloidal sphalerite variety used only rarely for cabochons. The first transparent wurtzite crystals were found in 2012, at the tanzanite deposit at Merelani Hills, south of Arusha, Tanzania (for identification, see www.rruff.info). The crystals, which measure up to 2 cm in diameter, are very rare. They have a tabular shape, with pyramidal faces. Some crystals from Merelani have clean parts, and several gemstones were faceted from them (figure 1).

Eight cut stones weighing 0.30–1.74 ct were studied. They were orange-brown, with a submetallic luster and  very weak, almost invisible, pleochroism. Refractive indices of these stones were well over the refractometer limit, listed as 2.36 and 2.38 in the literature (J. Bernard and J. Hyršl, Minerals and their Localities, Granit, Czech Republic, 2004, p. 666). They had very low birefringence, confirmed by a lack of doubling of facet edges. The determined specific gravity values of two of the crystals were 3.78 and 3.85; their measured hardness was 3.5. The stones were uniaxial positive and inert under both long- and short-wave UV. Their UV-Vis-NIR spectrum (figure 2) exhibited a high cut-off at 545 nm. The stones showed anisotropic crystals and hexagonal growth structures when viewed under a microscope. Because of the rarity and high value of the Tanzanian crystals, the faceted wurtzites are sure to catch the attention of gemstone collectors.

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Figure 2. The UV-Vis-NIR spectrum of a 1.74 cut wurtzite from Tanzania.

Jaroslav Hyršl is a mineralogist in Prague.