Hornblende is the most common member of a chemically complex mineral group called amphiboles. Amphiboles are minerals that contain ions of iron and/ or magnesium in their structures. Hornblende is in the same group as pyroxenes and biotite which are known as dark silicate minerals.

Hornblende is usually dark green to black and looks very splintery. It is predominantly found in continental rock where it makes up the dark portion of an otherwise light colored rock.

Information provided by Patrick M. Colgan, Northeastern University
http://www.casdn.neu.edu