The composition of staurolite is FeAl4[Si4]2O(OH)2, and it is a product of regional metamorphism. It may contain manganese or cobalt. It crystallizes in the pseudo-orthorhombic crystal system, and the crystals are prismatic. Staurolite is often twinned with a cross-shape or ‘X-shape’, from which it was given the name ‘cruciform stone’. It can be brown or brownish black, and it has a specific gravity of 3.65–3.77 and a Mohs hardness of 7–7.5. It often occurs in crystalline schist and in placer deposits.