Fabled Viking 'sunstone' shown to really work

"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.""