Interesting story here about the Norse use of a special crystal, ON
"sólarsteinn' "sunstone", to locate the sun in cloudy weather.,,2007399,00.html

A follow-up to earlier research by Horváth:

For more background see:

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:
Also six references to 'sólarsteinn' in church inventories (máldagar)

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 [ ], actually describes the
stone's use.