From the New Scientist, 31 Jan 2011:
Viking sagas may have been more truthful than we realised. Crystal “sunstones” could have helped Viking sailors to navigate even when cloud or fog hid the sun.
Vikings navigated using sundials calibrated to show the direction of the North Pole. While there is no physical evidence for the navigational techniques adopted on cloudy days, there are references in the Viking sagas to “sunstones” being used.