Staurolite is a red 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 and a rather complex chemical formula: (Fe,Mg,Zn)2Al9(Si,Al)4O22(OH)2. Iron, magnesium and zinc occur in variable ratios.
A special property of staurolite is that it often occurs twinned in a characteristic cross-shape. The macroscopically visible crystals are of prismatic shape. They are often larger than the surrounding minerals and are then called porphyroblasts.
The name is derived from the Greek, stauros for cross and lithos for stone in reference to the common twinning. Staurolite is a regional metamorphic mineral of intermediate to high grade. It occurs with almandine garnet, micas, kyanite and other metamorphic minerals.
It is the official state mineral of Georgia.