Iceland spar
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Iceland spar, formerly called Iceland crystal and also called optical calcite, is a transparent variety of calcite, a crystallized calcium carbonate, originally brought from Iceland and used in demons
Calcite rhombohedral crystal structure
Calcite crystal birefringence
Iceland spar, possibly the Icelandic medieval sun stone used to locate the sun's direction in overcast or foggy skies
Birefringence
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Birefringence, also called double refraction, is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically aniso
Iceland spar
…calcium carbonate (CaCO3). It crystallizes in the trigonal system, typically forming rhombohedral crystals. It has a Mohs hardness of 3 and exhibits double refraction, splitting a ray of light into two rays that travel at different speeds and directions. Iceland spar forms in sedimentary environments, mainly limestone…
A calcite crystal laid upon a graph paper with blue lines showing the double refraction
View from under the Sky Pool, London with coloured fringes due to stress birefringence of partially polarised skylight through a circular polariser
Sandwiched in between crossed polarizers, clear polystyrene cutlery exhibits wavelength-dependent birefringence
Color pattern of a plastic box with "frozen in" mechanical stress placed between two crossed polarizers