gems, corundum, ruby, Mozambique.12, Madagascar.12, inclusions.5, zircon, primary rutile

In August 2022, a team of GIA gemologists visited the ruby mines near Montepuez in the Cabo Delgado province of northern Mozambique (Fall 2022 GNI, pp. 383–386). This included a visit to the deposits farther west in the region, near the village of Caraia. This mining area is north of the N14 road, while most of the previously known productive ruby deposits are southeast of this highway. Ruby mining near Caraia has been ongoing for more than a decade but was mostly limited to artisanal mining activity until 2017. In the last few years, the ruby deposits have been developed on a larger scale by Fura Gems. The first official sale of this material took place in late 2021, and regular auctions have been organized since then (Fall 2021 GNI, pp. 276–277). During GIA’s visit in August 2022, gemologists collected a large suite of rough rubies directly from the wash plant and sort house at the mine site. The samples ranged from 0.1 to 1.2 g and were transparent and fracture-free. The vast majority contained some crystal inclusions, and an extensive inclusion study of this material matched well with the known data from Mozambican ruby, with one exception. The most common inclusions are bands of fine silk, short needles, and particles, including reflective platelets. Mozambican rubies typically show three distinctive types of solid crystals: transparent greenish amphibole crystals with a stubby to elongated shape; pseudohexagonal frosty mica crystals with small expansion fractures; and black opaque sulfide crystals with a metallic luster. All of these features were prevalent in the rubies mined near Caraia. However, one observation stood out: in 8 of the 85 studied samples, zircon crystal inclusions were found. While zircon crystals are also transparent, their relief is higher than that of the more common amphibole crystals. They also tend to be smaller and are rarely elongated in rubies. The most efficient way to identify them is by viewing the stones between crossed polarizers, which highlights the obvious stress halos around the slightly radioactive zircon crystals (figure 1). In all of GIA’s samples, the identification was also confirmed by confocal Raman spectroscopy. To our knowledge, zircon crystals have not been previously documented in rubies from Mozambique. They are, however, a hallmark for rubies from Madagascar (A.C. Palke et al., “Geographic origin determination of ruby,” Winter 2019 G&G, pp. 580–613). The presence of zircon crystal inclusions is often a decisive discriminator in the origin determination of rubies. The inclusion scenes of high-quality Mozambican and Malagasy rubies can look very similar, but only the latter were known to have abundant zircon inclusions. This is no longer true now that zircon inclusions have also been identified in rubies from Mozambique.