'Iceland spar' definitions:

Definition of 'Iceland spar'

(from WordNet)
noun
A transparent calcite found in Iceland and used in polarizing microscopes

Definition of 'Iceland spar'

From: GCIDE
  • 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]