The composition of hornblende is NaCa(Mg,Fe,Al)2[(Si, Al)4O11]2(OH)2. It crystallizes in the monoclinic crystal system. Its cross-section is nearly rhombic hexagonal, and crystal twinning is common. The aggregates are usually blocky with dense, granular, columnar, bladed and fibrous forms. The colour is green, greenish brown or black with white or grey streaks. Its diaphaneity is translucent to opaque, and its lustre is vitreous. The two cleavage angles are 60° and 120°. The specific gravity is 3.28–3.41, the Mohs hardness is 5–6, and the fractures are uneven. Hornblende is a major rock-forming material of intermediate igneous rocks, hornblende schist, amphibole gneiss and amphibolite. It is also found in granite, gabbro and alkaline rocks.