The Vikings were fabled to have owned a crystal that helped them navigate the seas. This was, of course, before magnetic compasses were invented. Few believed the tale. Now researchers in France seemed to have dug up a stone (pictured) which they believe was a sunstone.
The stone is a crystal of Iceland spar, a form of calcite that detects polarised light. Sunlight is polarised (that is its electromagnetic waves can travel in a particular orientation), which when passed through the sunstone could reveal the direction from which it came. Even if there was thick cloud cover, sunstones were able to reveal sun’s direction, making the Vikings’ lives easier.
PS: The stone pictured is not transparent because it has aged.
Reference: Le Floch et al. Proceedings of the Royal Society (2013) http://dx.doi.org/10.1098/rspa.2012.0743
Further reading: The Economist