LONDON – A rough, whitish block recovered from an Elizabethan shipwreck may be a sunstone, believed by some to have helped Vikings and other medieval seafarers navigate the high seas, researchers say.

In a paper published this week, a Franco-British group argued that the Alderney Crystal – a chunk of Icelandic calcite found amid a 16th century wreck at the bottom of the English Channel in 2002 – worked as a kind of solar compass, allowing sailors to determine the position of the sun even when it was hidden by heavy cloud, masked by fog or below the horizon.

That’s because of a property known as birefringence, which splits light beams in a way that can reveal the direction of their source with a high degree of accuracy. Vikings may not have grasped the physics behind the phenomenon, but that wouldn’t present a problem.

“You don’t have to understand how it works,” said Albert Le Floch, of the University in Rennes in France. “Using it is basically easy.”

Vikings were expert navigators – using the sun, stars, mountains and even migratory whales to help guide them across the sea – but some have wondered at their ability to travel the stretches of open water between Greenland, Iceland and Newfoundland in modern-day Canada.

Le Floch is one of several who have suggested that calcite crystals were used as navigational aids for long summer days in which the sun might be hidden behind the clouds. He said the use of such crystals may have persisted into the 16th century, by which time magnetic compasses were widely used but often malfunctioned.

Le Floch said one reason no stones have been found before is that calcite degrades quickly – it’s vulnerable to acid, sea salts and heat. The Alderney Crystal was originally transparent, but the sea water had turned it a milky white.