"These sunstones are mentioned in several contemporary texts, and were said to work even when the sky was completely overcast or the sun was below the horizon - as it is for long periods at such northern latitudes.
"The weather was thick and stormy... The king looked about and saw no blue sky," reads the 13th-century Hrafns Saga. "Then the king took the sunstone and held it up, and then he saw where [the sun] beamed from the stone.""