By the light of the sun

Vikings, as everybody knows, got around a bit. By sailing, largely. They are thought to have reached America, long before it was being called that, they made themselves busy round most of the European coastline and reached the Mediterranean, the Black and the Caspian Seas. They knew how to handle a boat.

This all took place before there were magnetic compasses to help them navigate, so it's all the more remarkable. I had a customer come into the shop last week looking for a piece of iolite, a blue-violet variety of cordierite. They wanted it because they knew it as Viking's Compass - apparently it had been used by Vikings to find the sun on overcast days. Iolite is pleochroic; the colour varies as you turn the stone in the light. This happens as light of different polarizations is bent to different degrees by the mineral structure as it passes through. I'd not heard of this use of the stone before, and also hadn't heard of an iolite source in Scandinavia. It's not a hugely rare stone, however, and I mentioned above, these were some well-travelled guys.

In the news today, though, is an article about a cleavage rhomb of Iceland Spar - or optical calcite, above right - found on an Elizabethan ship sunk in 1592. It looks like it had been used as a navigation aid in a manner similar to that mentioned above. This clear form of calcite has a set of recurring planes of weaknesses - cleavages - which cause it to break into distinct rhombic shapes. It's known for its birefringence, where light passing thought the rhomb will produce a double image. Another way to find the sun, by rotating the stone until the images are of the same intensity. This find, though dating to a few hundred years after the Vikings had calmed down somewhat, adds weight to the belief that they used some form of crystal to find the sun and, subsequently, their direction. And it'd have been pretty easy for Vikings to get their hands on samples of Iceland spar. Somewhere or other.