Critical minerals containing elements such as tantalum and niobium have always been in the news, and they are essential for many specialised applications in technology, especially electronic components, mobile phones and special metal alloys. They have become indispensable world-wide.

These mineral ores are usually in unattractive opaque masses or crystals, but rarely they can be transparent and suitable for faceting as rare gemstones.

We have recently been fortunate in acquiring two examples of these rare mineral species faceted as gemstones, Stibiotanatalite, antimony, tantalum, niobium oxide, and Tantalite-(Mn), manganese, tantalum oxide. The Stibiotantalite of 0.86 carats weight (0.17 grams) is lemon yellow, and the Tantalite-(Mn) of 1.19 carats (0.23 grams) is an attractive red. Because the gem-quality forms of these minerals are so rare and in such short supply world-wide, only small gems can be fashioned from suitable material. They are particularly difficult to cut as they can easily cleave and ruin the stone.

The Tantalite-(Mn) has a fancy cushion cut and the Stibiotantalite has a rectangular emerald cut. Both are from classic locality of Alto Lighoña in Mozambique, and together make a nice contrasting pair.

Tantalite-(Mn) (1.19 carats)
Tantalite-(Mn) (1.19 carats). Image: Monique Meredith
© Australian Museum

This acquisition fulfils part of our gemstone collection strategy: that of acquiring rare and unusual gems faceted from ore minerals commonly in opaque, massive form, not usually of gem quality. The acquisition augments an existing set of such rare gemstones, with Cerussite (lead carbonate), Anglesite (lead sulfate), Crocoite (lead chromate), Cuprite (copper oxide), Cassiterite (tin oxide), and Sphalerite (zinc sulfide), already in our collection and currently all on display in the Minerals gallery.

These specimens also have a detailed history and provenance, which is rare for faceted gems. The original crystals came from a collection owned by Walter Oberholzer, legendary Professor of Mineralogy at ETH University of Zurich, Switzerland, who obtained them when he was in Mozambique just after World War II. The faceting was done by internationally famous Gemmologists David and Maria Atkinson, specialists in rare and difficult to facet gemstones, who acquired the original gem-quality crystals from Prof. Oberholzer in 1985 and made the cut stones available. The Atkinsons are big fans of the Australian Museum and its Mineralogy section and live part of the year in Western Australia with their main home in Arizona. They have a good eye for the kinds of special and unique specimens we are always looking for and alert us when something outstanding is available at reasonable cost. We have benefited from their expertise for several past gemstone acquisitions, now on display in the Minerals gallery.

Stibiotantalite (0.86 carats)
Stibiotantalite (0.86 carats). Image: Monique Meredith
© Australian Museum

These minerals have special optical properties, having high refractive index and strong dispersion, so they show a lot of “fire” or sparkle in their refracted light response, making them attractive display and study specimens. Crystals of these minerals of sufficient size and clarity to facet are rare indeed. I was fortunate to source these gems which highlight a very different aspect of critical minerals.

The purchase was facilitated by funding from the Australian Museum Research Institute.