This is quite possibly factual legend more than mythological lore. The voyaging Vikings would have to deal with crazy weather in the north and the violent seas of the god Njord.
Legend tells of a stone they would hold up to the sky and look through. It would refract the light and act as a polarizing filter that would allow them to see where the sun was through cloudy skies. Leaving them a way to navigate the stormy seas.
This gemstone is commonly known as Iolite and is a most desirable rich violet-blue. Its name even comes from the Greek word for violet. But it also goes by the name “Viking Compass”.