At http://phys.org/print388825389.html …. another study on the Viking sun stones – this time a proper experiment …
…. although sun stones have been featured in a couple of earlier News reports this one has an image that sort of clarifies what is being put forward. The North Atlantic is notoriously cloudy so how did the Vikings navigate? This piece of feldspar was used to see if the idea of a sun stone was realistic. Apparently it worked. The feldspar came from Iceland but presumably similar stone was available in Scandinavia.