- a mineral, basic iron aluminum silicate, Fe2Al2O7(SiO4)4(OH), occurring in brown to black prismatic crystals, which are often twinned in the form of a cross.
- a brown glassy mineral consisting of iron aluminium silicate in the form of prismatic crystals: used as a gemstone. Formula: Fe 2 Al 9 Si 4 O 11 (OH) 2
