Study of physically worn brooches shot through refractional calcite (Iceland spar)

Note:
It has been speculated that the sunstone (Old Norse: sólarsteinn) mentioned in medieval Icelandic texts was Iceland spar and that Vikings used its light-polarizing property to tell the direction of the sun on cloudy days, for navigational purposes.

Historically, the double-refraction property of Iceland spar was important to understanding the nature of light as a wave. This was studied at length in the 17th century AD by Christiaan Huygens and Isaac Newton.