A common mineral embracing many varieties varying in
   color and in composition. It occurs in monoclinic crystals; also
   massive, generally with fibrous or columnar structure. The color varies
   from white to gray, green, brown, and black. It is a silicate of
   magnesium and calcium, with usually aluminium and iron. Some common
   varieties are tremolite, actinolite, asbestus, edenite, hornblende (the
   last name being also used as a general term for the whole species).
   Amphibole is a constituent of many crystalline rocks, as syenite,
   diorite, most varieties of trachyte, etc. See Hornblende.