Saw this a while back, thought you, in particular, would be interested: Crystals may have aided Viking Sailors and Crystals Helped Viking Sailors.

Seems the Vikings used a variety of calcite known as Iceland spar, a clear rhombahedral crystal that nicely splits light into two directions and speeds, to find the direction of the sun on cloudy days. It was also from these crystals that much of the understanding of mineral optics originated, of which polarization is key.

(Iceland Spar is also the name of a chapter in Pynchon's latest novel, which I recently finished and quite enjoyed)

I still have the spar that I got when I was a child, but it is very scratched. After reading the book I had to stop by a local rock shop and pick up four nice crystals ($10) just to play 'round with on the desk when I should be working.