Viking sunstone, a legendary crystal that could detect the location of the Sun on cloudy days. This, of course, would be a boon to navigation in the age before GPS.
Sunstones are mentioned in some Viking sagas and historians have theorized they were double-refracting crystals such as cordierite, tourmaline, or calcite, which are common in Scandinavia. These crystals only allow light through them that's polarized in certain directions and thus appear darker or lighter depending on the polarization of the light behind it. While the Sun may be blocked by clouds, it's still sending out a concentration of polarized light that can be detected by the crystal as it's moved around.
But that's only a theory. No sunstone has ever been found. . .until now.
A team of French archaeologists studying artifacts from a British ship that sunk in 1592 found a rectangular block of Iceland spar calcite crystal, a type known for its double-refracting properties. The wreck was near Alderney island in the English Channel.
You can see the sunstone in this picture, next to a pair of dividers that may have been used for navigation. Both items were found close together in the wreck.
It's interesting that the sunstone was found on a ship dating centuries after the Viking era. It looks like these things were more popular than anyone ever suspected.
Photo courtesy Alderney Society.