sunstone

Short definition from Wiktionary

Noun

1. 1
   A translucent form of feldspar having flakes of hematite, used as an ornamental stone.
2. 2
   A stone, possibly Icelandic spar, said in late medieval sources to have been used by navigators in the Viking Age and later to find the location of the sun in an overcast sky, possibly via polatization of light.