Hornblende, the most common form of a group of silicate minerals called amphiboles. Hornblende has a complex composition, containing varying amounts of calcium, sodium, iron, magnesium, and aluminum. Hornblende is typically dark green to black. It usually occurs as prism-shaped crystals in granite, schist, gneiss, or basalt.
Hardness: 5 to 6. Specific gravity: 3 to 3.5.