Twinned staurolite crystals. Staurolite is a hydrated aluminium silicate mineral that contains iron, magnesium and zinc in variable ratios. The colour varies from red-brown to black, and it can also have white streaks in it. It occurs in metamorphic rocks. The crystal structure is monoclinic and it usually forms prism-shaped crystals, though here the crystals are twinned. Crystal twinning occurs when two crystals grow together, forming an interlocked twinned structure. These samples are examples of penetration twinning.