Merelaniite in Gem Diopside from Merelani, Tanzania
A recent Micro-World entry (Summer 2018 G&G, pp. 226–227) described merelaniite occurring as dense inclusions in a tanzanite cabochon and as inclusions in two gem tanzanite crystals. While the Merelani tanzanite mines have also become well known for green diopside in recent years, in July 2019 the authors observed, for the first time, dense inclusions of fine merelaniite whiskers in gem-quality yellow-green, prismatic diopside crystals (figure 1, left).
Seven merelaniite-included diopside crystals, reportedly mined in 2018 and obtained by one of the authors (WR), ranged in size from approximately 1 to 5 cm long. Apart from the merelaniite inclusions, all but the largest had excellent clarity, although one 1.7 cm crystal was so densely filled with merelaniite that half of the sample appeared very dark. Similar to the inclusions observed in tanzanite, the merelaniite inclusions in diopside were predominantly cylindrical whiskers with metallic luster and variable diameters and lengths. The longest whiskers in the largest diopside spanned up to 2 cm (figure 1, top right), and many extended to the surface, where they terminated. Some of the merelaniite whiskers showed a spindle- or wheel-type structure with diameter that varied along the axis of even a single whisker (figure 1, bottom right). Some ribbon-like inclusions also occurred that were likely merelaniite. All of the diopsides were also associated with white spheroidal clusters of radiating acicular mesolite crystals, while several crystals were associated with graphite and one was associated with a transparent blue fluorapatite crystal.
While reviewing an earlier version of this report, GIA’s Nathan Renfro made the connection between these included diopside crystals and a faceted Merelani diopside gemstone (figure 2, left) containing dark whisker inclusions. Author JIK obtained that diopside from Bill Vance, the stone’s cutter, in approximately 2012. The whiskers in this 1.35 ct stone, some of which were surrounded by remarkable dark circular growths (figure 2, right), have also been confirmed by Raman spectroscopy as merelaniite.
