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.