The BBC article mentioned refraction, but Iceland Spar makes it possible to see the direction of polarization of skylight, which means it could be used on small, clear patches of sky on an otherwise overcast day to find the direction to the sun. This is another way it could act as a "sunstone" for navigation. Whether it actually was used this way is unfortunately not known.
I love when believed 'mythology' is shown to be fact. We write off our ancestors too often and they were great hackers in their own right.<p>Now the Mythbusters just need to get Archimedes mirror working one day!
In the book 'Emergency Navigation' by David Burch ( <a href="http://www.amazon.com/Emergency-Navigation-Improvised-No-Instrument-Methods/dp/0071481842" rel="nofollow">http://www.amazon.com/Emergency-Navigation-Improvised-No-Ins...</a> ), the method is explained more clearly. With a clear Iceland spar crystal 1 to 2 inches on a side, the direction of the sun can be pinpointed to within a few degrees. You can also get the same effect using a piece of cellophane and the lens of a pair op polarized sunglasses. You do need a patch of clear sky at a 90 degrees angle from the sun direction to observe the change in intensity.
This is kind of old news. This Ouest-France article from November 5th, 2011 even has two pictures of the 'Viking sunstone'<p><a href="http://www.ouest-france.fr/actu/actuLocale_-Des-physiciens-percent-le-secret-des-Vikings-_-2007955------35238-aud_actu.Htm" rel="nofollow">http://www.ouest-france.fr/actu/actuLocale_-Des-physiciens-p...</a>
A little off topic, but Iceland Spar features prominently in Pynchon's "Against the Day"[1]. It is a pretty amazing book that might appeal to this audience.<p>[1] <a href="http://en.wikipedia.org/wiki/Against_the_Day#Doubling" rel="nofollow">http://en.wikipedia.org/wiki/Against_the_Day#Doubling</a>