A crystal found in a shipwreck could be similar to a sunstone – a mythical navigational aid said to have been used by Viking mariners, scientists believe.
The team from France say the transparent crystal may have been used to locate the Sun even on cloudy days.
However, a number of academics treat the sunstone theory with scepticism.
The team from the University of Rennes in France say they found the crystal while examining the wreck of a British ship sunk off the island of Alderney – in the English Channel – 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 has now been shown that it is of 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.