Noun
A mineral of a brown to black color occurring in prismatic crystals, often twinned so as to form groups resembling a cross. It is a silicate of aluminia and iron, and is generally found imbedded in mica schist. Called also granatite, and grenatite.
Source: Webster's dictionaryA subgroup is the andalusite -, staurolite -, kyanite - and sillimanite -schists which usually make their appearance in the vicinity of gneissose granites, and have presumably been affected by contact metamorphism. Source: Internet
Penetration twins consist of two single crystals that have grown into each other; examples of this twinning include cross-shaped staurolite twins and Carlsbad twinning in orthoclase. Source: Internet