Interesting story here about the Norse use of a special crystal, ON
"sólarsteinn' "sunstone", to locate the sun in cloudy weather.

http://www.guardian.co.uk/science/story/0,,2007399,00.html

A follow-up to earlier research by Horváth:

http://www.opticsinfobase.org/abstract.cfm?URI=JOSAA-22-6-1023

For more background see:

http://www.polarization.com/viking/viking.html
http://www.raunvis.hi.is/~thv/t_t.html

The Old Norse sources are:

Rauðs þáttr (Rauðúlfs þáttr) in Óláfs saga helga.
Hrafns saga Sveinbjarnarsonar.
Guðmundar saga (in a passage based on Hrafns saga).
The Icelandic translation which is all that survives of Abbot
Arngrím's Life of Guðmund; here the stone is called a crystal:
'kristallus'.
Also six references to 'sólarsteinn' in church inventories (máldagar)
1318-1408

See Foote 1956: 'Icelandic Sólarsteinn and the medieval background',
Arv XII, reprinted with a postscript that discusses the polarisation
theory in Aurvandilstá: Norse Studies.

Only Rauðs / Rauðúlfs þáttr, quoted in the article above [
http://www.raunvis.hi.is/~thv/t_t.html ], actually describes the
stone's use.