Bixbyite is a manganese iron oxide mineral with the chemical formula (Mn,Fe)2O3. This is a superb 11mm metallic black crystal, with mirror-like luster, well exposed and beautifully perched on a matrix of rhyolite, with little topaz crystals perched on top. The modified cubic crystal shows octahedral faces on its corners and along edges, indicative of twinning. This is one of the finest bixbyite thumbnails I have seen and is top of its class in sharpness, luster and composition. No damage.