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Crystals like calcite, also called Iceland spar, can reveal the direction of polarization, a bit like a prism that reveals the rainbow within white light.

Light from the sky is polarised and, as he discovered in 2011, looking through a piece of Iceland spar reveals the direction of polarisation, and thus the direction of the sun, to within 5°.

Dove's Iceland spar prism is also useful, when two pigments have to be worked on to paper, so as to be complementary.

Spar, sp�r, n. a term applied by miners to any bright crystalline mineral, and adopted by mineralogists in the names of a number of minerals—calcareous spar, fluor spar, Iceland spar, &c.—adj.

He and his colleagues have been experimenting with a mineral called Iceland spar.