Crystals 'helped Viking sailors'

Crystals 'helped Viking sailors' navigate even when the sun was obscured by clouds. The sunstones, or birefringence rocks, polarize the light into two distinct beams. Gotta love the ingenuity of figuring out how to use the beams to shine on a sundial to be used to figure out where they were and where they were going.

And I want one of these birefringent crystals. But I also want a solar oven too.