Source : Webster's Revised Unabridged Dictionary (1913)

Iceland spar \Ice"land spar`\ (Min.)
   A transparent variety of calcite, the best of which is
   obtained in Iceland. It is used for the prisms of the
   polariscope, because of its strong double refraction. Cf.
   {Calcite}.

Source : WordNet®

Iceland spar
     n : a transparent calcite found in Iceland and used in
         polarizing microscopes