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."
As far back as the 1960s, scientists suggested that the stones described could be Icelandic spar, or calcite. This shows birefringent and dichroic properties, changing color and brightness when rotated in front of polarized light.
While the light from the sun isn't originally polarised, it becomes so by the time it's passed through the atmosphere.
Many scientists believed the polarization effects of the crystals would be too small to be of any practical use. But the discovery of such a crystal on a 16th-century shipwreck in the 1970s gave a University of Rennes team the chance to check.
While such a stone might seem redundant after the invention of the magnetic compass, says the team, the iron in the ship's cannons would have made a compass unreliable.
Simply rotating the crystal gave a good chance of detecting the sun, the team found. But when the stone was covered with an opaque sheet with a hole at its center, two distinct shadows were created. Rotating the crystal now made one shadow lighter and the other darker, according to their angle to the sun.
The stone had to be 'calibrated' on a sunny day, but can then be used in much darker conditions, says the team.