Literally, "cross-stone"; a reddish-brown or black mineral of iron and magnesium that occurs as prismatic crystals in metamorphic rocks, often in a cross shape. It is used as a gemstone that often forms twin crystals in the shape of a cross. The term metamorphism refers to a change in the physical structure of rock that results from long-term heat and pressure; especially a change that increases the rock's hardness and crystalline structure.