Bixbyite is a manganese iron oxide mineral with formula: (Mn,Fe)₂O₃. The iron/manganese ratio is quite variable and many specimens have almost no iron. It is a metallic dark black with a Mohs hardness of 6.0 - 6.5. It is a somewhat rare mineral sought after by collectors as it typically forms euhedral isometric crystals exhibiting various cubes, octahedrons, and dodecahedrons.