A long, dark mineral with abundant cleavage faces in a granitic rock is hornblende more often than not. The chemical makeup of hornblende is quite variable, so its formula is ugly: (Ca,Na)2-3(Mg,Fe+2,Fe+3,Al)5(OH)2[(Si,Al)8O22].
Hornblende is usually black but can also be dark green or brown. It is a common primary mineral in granitic rocks and a common metamorphic mineral in gneiss and schist.