Iceland spar : 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.

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Taxonomy
Iceland spar, sunstone, free encyclopedia, novel, crystals, Viking, Ramón, Hegedüs, calcite, Iceland, carbonate mineral, search, refracting, Iceberg, claim, Dixon, New Mexico, pound, United States.

Defination from the web:
Iceland spar, formerly known as Iceland crystal, is a transparent variety of calcite, or crystallized calcium carbonate, originally brought from ... Phillip Pullman references the doubling property of Iceland spar in his 2000 novel The Amber Spyglass, the third volume in the His Dark Materials ...