Iceland spar

a transparent variety of calcite that is double-refracting and is used as a polarizer.

1820–30 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
Iceland spar
a pure transparent variety of calcite with double-refracting crystals used in making polarizing microscopes

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
Iceland spar   (īs'lənd)  Pronunciation Key 
A form of calcite that is transparent and causes light passing through it to refract in two directions (forming a double image of an object seen through it). Iceland spar occurs in perfect rhombohedrons and is used in optical instruments.
The American Heritage® Science Dictionary
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