"A stone with which one could see where the sun was in Heaven."

Here.

"Using his research and his navigational skills, Karlsen seeks to solve the mystery of how the Vikings could have determined the vital position of the hidden sun. In his book he writes: 'During the middle of the summer there is too much light in the night sky to see the stars at the high latitudes where the Vikings sailed. The sun was the only dependable celestial body available for reference.' He explores hints in the ancient Viking Sagas, as well as Thorkild Ramskou's theory of a mineral answer, by devising and successfully utilizing what he considers to be the most obvious choice: calcite.

"Karlsen makes a very strong case for Iceland Spar. His proposed technique is premised on the ready availability of optical quality calcite in an area of Iceland where the Vikings made first landfall and which relies on the mineral's high birefringence. Mr. Karlsen devised a plausible scenario as it might have happened more than 1000 years ago and which he proved to be extremely accurate.

"The Vikings reached their destinations by latitudinal sailing, that is to say they sailed in straight east-west courses. They used a myriad of navigational clues and techniques to hold to this kind of course, such as observation of sea-birds, waves, stars and the sun. Karlsen found that with the help of a calcite sunstone and a 'bearing board,' very accurate determinations of latitude could be obtained."

From here. More here.