Tetradymite is a very rare bismuth tellurium sulphide which is even rarer as discrete individual crystals which are usually trigonal or complex twins - the name being derived from four (tetra) and twins (dym). Župkov in Žarnovica County, Banská Bystrica Region, Slovakia is the Type Locality and is where some of the finest crystals have been found. Despite that, crystals are always very small. Remarkably, the locality of Župkov is only known for seven minerals, making it one of least diverse mineralogical sites to lay claim to a new species. One of those other species is Montanite, a secondary hydrated bismuth tellurate which forms as an alteration product associated with Tetradymite. This specimen has not been tested, but it is possible that the pale cream coloured coating covering the matrix might be Montanite. The Tetradymite crystals, which are richly scattered across the whole display surface, exceptionally reach 3 mm in size but most are around 1 to 2 mm. They display several forms with triangular (trigonal) surfaces dominant. This particularly fine specimen is from the superb systematic collection assembled by the brothers Martin and Michael Günther (1951-2007 & 1956-2021).
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