A team of French archaeologists believe they have found a sunstone, a strange crystal that was said to help mariners locate the sun even on overcast days.

Some of the medieval Norse Sagas mention this device. In "Rauðúlfs þáttr," King Olaf asks the hero Sigurður to point out the sun in the middle of a snowstorm. Sigurður points to where it is behind the gray sky. To test him, the king had a follower "fetch the sunstone and held it up and saw where light radiated from the stone and thus directly verified Sigurður's prediction."

One recent study suggests the "sunstone" was a double-refracting crystal, which allows light through when the light is polarized in certain directions. They brighten or darken depending on the polarization of the light behind it. Clouds block the sun's visible light but let through a concentration of polarized light that can be detected by the crystal as it's moved around. Double-refracting crystals such as cordierite, tourmaline and calcite are common in Scandinavia.

Some scholars have expressed doubts about the sunstone's existence because "Rauðúlfs þáttr" is a highly allegorical tale full of magical events.

Now it appears the tale may not be all that fantastic after all. Archaeologists from the University of Rennes have been studying finds from a British ship that sunk in 1592 near the island of Alderney in the English Channel. They found a rectangular block of Iceland spar calcite crystal, a type known for its double-refracting properties. The crystal was found next to a pair of dividers that may have been used for navigation.

The researchers suggest that their discovery shows the use of sunstones lasted well beyond the Viking era.

The team's results appear in the latest issue of the "Proceedings of the Royal Society."


[Photo courtesy Alderney Society Museum]

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