French scientists have found a crystal which could be similar to a sunstone - a mythical navigational aid said to have been used by the Vikings.
The crystal may have been used to locate the sun even on cloudy days and could help explain how the Vikings were able to navigate across large tracts of the sea well before the invention of the magnetic compass.
The team from the University of Rennes said the crystal was found in the wreck of a British ship sunk in the English Channel off the island of Alderney in 1592.
An oblong crystal the size of a cigarette packet was next to a pair of dividers - suggesting it was part of the navigational equipment.
It is Iceland spar - a form of calcite known for its property of diffracting light into two separate rays.
Testing a similar crystal, the scientists proved that by rotation it was possible to find the point where the two beams converge - indicating the direction of the Sun.
They say it works on cloudy days, and when the Sun has set.
However, the BBC reports a number of academics treat the sunstone theory with scepticism.
This particular piece of Iceland spar was found on the British vessel long after the Viking heyday in the ninth and 10th Centuries.
But the scientists conjecture that use of sunstones may have persisted for many centuries as a back-up to the often unreliable magnetic compass, which was introduced in Europe in the 13th Century.
There is only a sketchy reference to the sunstone in Viking legends.