PARIS: An oblong crystal found in the wreck of a 16th-century English warship is, it turns out, a sunstone, a near-mythical navigational aid said to have been used by Viking mariners.

The stone, researchers said, is made of Iceland spar, a transparent, naturally occurring calcite crystal that polarizes light and can get a bearing on the Sun.

It was found in the remains of a ship that Queen Elizabeth I dispatched in 1592 as a precaution against a second Spanish Armada that foundered off the Channel island of Alderney.

A sunstone fractures light, enabling seafarers to locate the Sun even when it is behind clouds or has dipped below the horizon.

Long before the magnetic compass was introduced to Europe in the 13th-century, theory has it, sunstones helped Norse mariners navigate their way to Iceland, perhaps as far as North America, between 900 and 1,200 A.D.

Yet the idea has remained frustratingly without solid proof.

This stone is about the size of a small bar of soap whose edges have been trimmed at an angle. Using a transparent crystal similar to the original, scientists were able to follow the track of the setting sun in poor light, with an accuracy of one degree. In a second experiment, they were able to locate the sun for 40 minutes after sunset.

Put together, these suggest the sunstone may have been kept as a backup to a magnetic compass.