A gemstone with just one known specimen

Not the usual thing I comment on, but I find it really interesting that there is a naturally-occurring mineral out there with just a single known specimen in the entire world.

The awkwardly-named kyawthuite (actually named after Dr. Kyaw Thu, a former Myanmar geologist) was discovered near Mogok, Myanmar, in 2015, picked up by sapphire hunters from a remote streambed. This specimen of the pretty reddish-orange gemstone is tiny, just 1.61 carats (about 0.3 grams), but it has never been found anywhere else, which makes it pretty special (and valuable).

Kyawthuite is mainly composed of the minerals bismuth and antimony, with traces of tantalum, and has a chemical formula Bi3+Sb5+O4. None of these constituent elements are particularly rare in themselves, but the mixture was subjected to immense pressures and heat when India crashed into the rest of Asia (the geological event that pushed up the Himalayas). The resulting mineral is therefore extremely dense, eight times that of water. It was recognized by the International Mineralogical Association in 2015, and its scientific description was published in 2017.

Interestingly, another very rare mineral, intriguingly called painstone, was also discovered in Myanmar, and it too is only represented by a handful of known specimens. There may well be many more, but Myanmar is a notably difficult place to explore, what with ongoing wars and international sanctions.