Staurolite

Staurolite is a reddish brown to black, mostly opaque, nesosilicate mineral with a white streak. It crystallizes in the monoclinic crystal system, has a Mohs hardness of 7 to 7.5. Magnesium, zinc and manganese substitute in the iron site and trivalent iron can substitute for aluminium.

Staurolite often occurs twinned in a characteristic cross-shape, called cruciform penetration twinning.

The name derived from the Greek, stauros for cross and lithos for stone in reference to the common twinning.