Staurolite
Twinning
Staurolite, a silicate, is used in geology as an 'index mineral'. This means it is used to estimate depth, temperature and pressure changes to rock, like movement of the earth’s crust or magma activity. Staurolite often occurs as twinned, six sided crystals, which can look like a cross. The word staurolite is from the Greek word ‘stauros,’ meaning cross.