Icelandic spar

[John Pendergast]

http://www.guardian.co.uk/science/2011/nov/02/sunstones-vikings-navigate-america?fb=native&CMP=FBCNETTXT9038

"Writing in the journal Proceedings of the Royal Society A, a team led 
by Guy Ropars at the University of Rennes in Brittany describe tests on 
a a piece of Icelandic spar found aboard a sunken Elizabethan military 
vessel. The ship was discovered in the 1970s by a fisherman off Alderney 
in the Channel Islands.

Through a series of experiments, they found a different way to use the 
crystal to pinpoint the position of the sun. They covered the crystal 
with an opaque sheet that had a hole in the centre. When viewed through 
the hole, they noticed the crystal cast two distinct shadows. Rotating 
the crystal made one shadow get lighter as the other darkened and vice 
versa."