Staurolite - Fe²⁺Al₉Si₄O₂₃(OH)

Habit: Reddish to blackish brown or yellowish brown; rarely blue. Short to long prismatic crystals, commonly cruciform twins, or "fairy crosses." Subvitreous to resinous luster; transparent to opaque. Gray streak.

Environment: Characteristic of intermediate-grade metamorphism of aluminum-rich rocks, with staurolite­almandine-kyanite being a common assemblage. Also forms at lower metamorphic grades with chloritoid or in schists, emery deposits or metamorphosed ultrabasic rocks.

Etymology: From the Greek stauros, meaning "cross," in allusion to the common cruciform twins crystal formation.

The cruciform twins are sometime used as ornaments and amulets. Staurolite is an index mineral for interpreting metamorphic grades, since it forms only in a narrow range of pressures and temperatures.