iceland spar

from WordNet (r) 3.0 (2006)

Iceland spar
    n 1: a transparent calcite found in Iceland and used in
         polarizing microscopes

from The Collaborative International Dictionary of English v.0.48

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}.
   [1913 Webster]