Staurolite is iron aluminum silicate and a mineral that is often found in metamorphic rocks. It occurs in long prismatic crystals, such as the ones from Uri, Switzerland, but more often the crystals are 'twins', that is, two crystals intergrown along a specific plane of symmetry. Staurolite often forms twins that intergrow at right angles, forming a cross. These have been given the fanciful name 'fairy crosses'.