Amphibole refers to a family of silicate minerals forming prism or needlelike crystals. Amphibole minerals generally contain iron, magnesium, calcium and aluminum in varying amounts, along with water. Hornblende always has aluminum and is a most common dark green to black variety of amphibole; it, forms forming in many igneous and metamorphic rocks. Actinolite has no aluminum; it and is needle-shaped and light green. Blue amphibole contains sodium and, of course, is bluish in color.