Iceland spar is a form of calcite that splits light into two beams.

ECONOMIST: How Norsemen found their way round in cloudy weather

This particular piece of Iceland spar was found on the British vessel long after the Viking heyday in the ninth and 10th Centuries.

BBC: 'Viking sunstone' found in shipwreck

Several centuries underwater have rendered the Alderney crystal opaque, but Dr Ropars and his team are now examining it, and believe it may be Iceland spar.

ECONOMIST: How Norsemen found their way round in cloudy weather

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.

BBC: 'Viking sunstone' found in shipwreck

Spar, Iceland, Checkers and the Co-Op all sold the meal but it has now been removed from the stores.

BBC: Jersey health plans law change to challenge mis-labelling