Iceland spar

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A doubly refracting transparent calcite used in optical instruments.

from the GNU version of the Collaborative International Dictionary of English.

• (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A transparent form of calcite, used to demonstrate the polarisation of light.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a transparent calcite found in Iceland and used in polarizing microscopes