Crystals 'helped Viking sailors'
Vikings may have used a special crystal called a sunstone to help navigate the seas even when the sun was obscured by fog or cloud, a study has suggested.
Polarisation cannot be seen with the naked eye, but it can be viewed with what are known as birefringent crystals, or sunstones.
Birefringence, or double refraction, is the splitting of a light wave into two different components - an ordinary and an extraordinary ray.
The researchers found that the crystals could be used to find out where the sun was in the sky in certain foggy or cloudy conditions.
Thanks to ChumpFriend Bruce Grand Pre for the link....