Cool applied science: Calcite crystals split incoming light into two images whose relative brightness depends on the polarization of the light. As it happens, polarization is how you can find the exact location of the sun on a cloudy day; and the Norse sagas mention a “sunstone” which the Vikings used for navigation. Unfortunately no sunstones have been found in ancient Viking settlements or shipwrecks yet, but calcite occurs in Iceland and would be a great candidate.
Loki probably took them all and hid them somewhere. That guy is such a pain in the ass.
A 1988 paper from the University of Alaska Fairbanks concerning navigation by polarized light and the Pfund Sky Compass, invented in 1944.
The Viking sunstone was probably a natural crystal of cordierite. This mineral, a silicate of magnesium, aluminum, and iron, has some similarity to modern polarizing filters in that it absorbs light of different polarizations differently. In cordierite, however, the absorption also depends on the color of the light. A natural crystal of this mineral looks somewhat like a quartz crystal, but would rarely be as large as the size of your little finger. The stone absorbs blue light more strongly than yellow if the light is polarized along the long axis of the crystal, and yellow light more than blue if the polarization is across the long axis. If the crystal is held overhead and rotated, its color changes from yellow to blue when the long axis is pointed toward the sun. Clouds on the horizon cause no problems, as long as the sky directly overhead is clear.
The modern sky compass is based on man-made polarizing filters. It started with the work of Dr. A. H. Pfund, who was studying the polarization of scattered light from the sky in 1944. Commander T. D. Davies of the U.S. Navy, who heard of the device three years later, was well aware of the problems of twilight navigation, and was instrumental in getting four experimental instruments made for the Navy. Many of the Air Force’s Ptarmigan flights to the North Pole in the 1950s used the sky compass.
SAS made further improvements and used the instrument for many years on their polar flights. The sky compass had a star- or cross-shaped insert of a material that rotated the plane of polarization of the incoming light, as well as a polarizing filter. When the instrument was properly aligned with the hidden sun, the cross disappeared.
Calcite is one of the most plentiful rocks on earth and occurs absolutely everywhere. There is probably some within 100 feet of you right now.
This fun fact brought to you by my otherwise useless Geology degree.