A transparent, colorless calcite, found esp. in Iceland: it is used by opticians for making double-refracting prisms.
Webster's New World
Iceland Spar Sentence Examples
He discovered double refraction in Iceland spar (Experimenta crystalli islandici disdiaclastici, Copenhagen, 1669).
Examining the light reflected from the windows of the Luxemburg palace with a doubly refracting prism, he was led to infer (though more refined experiments have shown that this is not strictly the case) that light reflected at a certain angle, called the polarizing angle, from the surface of transparent substances has the same properties with respect to the plane of incidence as those of the ordinary stream in Iceland spar with respect to the principal plane of the crystal.
Elliptically polarized, partially elliptically polarized and partially plane polarized light give with Iceland spar two streams of, in general, unequal intensity, neither of which can be made to vanish.