Braunite
Braunite is a silicate mineral containing both di- and tri-valent manganese with the chemical formula: MnMn6[O8|SiO4]. Common impurities include iron, calcium, boron, barium, titanium, aluminium, and magnesium.
Braunite forms grey/black tetragonal crystals and has a Mohs hardness of 6 - 6.5.