An oblong crystal found in the wreck of a 16th-century English warship is a sunstone, a near-mythical navigational aid said to have been used by Viking mariners, researchers said yesterday.
The stone is made of Iceland spar, a transparent, naturally-occurring calcite crystal that polarises light and can get a bearing on the sun, they said.
It was found in the remains of a ship that had been dispatched to France in 1592 by Queen Elizabeth I as a precaution against a second Spanish Armada but foundered off the island of Alderney, in the Channel.
British and French scientists have long argued that the find is a sunstone - a device that fractures light, enabling seafarers to locate the sun even when it is behind clouds or has dipped below the horizon.
Sunstones, according to a theory first aired 45 years ago, helped the Vikings navigate their way to Iceland and even perhaps as far as North America during the Viking heyday of AD900 to AD1200, way before the magnetic compass was introduced in Europe in the 13th century.
But there is only a sketchy reference in ancient Norse literature to a "solarsteinn", which means the idea has remained frustratingly without solid proof.
In a study published in the British journal Proceedings of the Royal Society A, investigators conducted a chemical analysis on a tiny sample, using a device called a spectrometer, which confirmed that the stone was a calcite.
The stone is about the size of a small bar of soap whose edges have been trimmed at an angle.