The Collaborative International Dictionary of English v.0.48:

Hornblende \Horn"blende`\, n. [G., fr. horn horn + blende
   blende.] (Min.)
   The common black, or dark green or brown, variety of
   amphibole. (See Amphibole.) It belongs to the aluminous
   division of the species, and is also characterized by its
   containing considerable iron. Also used as a general term to
   include the whole species.
   [1913 Webster]

   Hornblende schist (Geol.), a hornblende rock of schistose
      structure.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Amphibole \Am"phi*bole\ ([a^]m"f[i^]*b[=o]l), n. [Gr.
   'amfi`bolos doubtful, equivocal, fr. 'amfiba`llein to throw
   round, to doubt: cf. F. amphibole. Ha["u]y so named the genus
   from the great variety of color and composition assumed by
   the mineral.] (Min.)
   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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hornblende
    n 1: a green to black mineral of the amphibole group; consists
         of silicates of calcium and sodium and magnesium and iron