Viking Navigation
(or emergency compass)
The Vikings used iolite, the gem variety of the mineral cordierite, to assist navigation on a cloudy day when the sun was not visible.
Clouds scatter light, and scattered light can become polarized in certain directions. The light coming from the sun directly toward you on a cloudy day is unpolarized, but the light coming at you from a direction at right angles to that path is strongly polarized. Iolite has a particular crystal axis along which the polarized light is extinguished, where it looks dark.
To locate the sun, a Viking navigator would look through an iolite crystal while rotating it to find the direction where the light would not extinguish. That would be the direction of the unpolarized light - the sun.
The Vikings obtained their iolite from a deposit in Denmark. You have to wonder how such things were discovered. These days, we know about polarized light and polarizers, but would you have thought to hold a piece of strange stone up to the sky to help you find your way on an overcast day?
The Iolite does not have to be gem quality, but light has to be able to pass through it. It would be advisable to practice using the stone on a cloudy day in an area where you know where the sun is likely to be at any time of the day.
Suggestion - Once you have your Iolite Compass working, mount the iolite in a hole in a piece of cardboard or thin ply. This will cut the glare when holding the stone to the sky, make it easier to hold and easier to find if you drop it - BUT, make sure you orientate the stone in the card so that it works before glueing it in place.
Next time you’re in the bush on an overcast day and the iron ore deposits make your compass useless and the batteries die in your GPS, pull out your trusty Iolite Compass - no batteries, no GPS signal scrambling, no magnetic problems, works almost instantly.
PS - Might work at night with a full moon and light overcast. No moon, go to bed.
Heavy overcast, find high ground, under cover, go to bed.