Medieval Icelandic texts indicate that the Vikings used the spar, which they called the Sunstone, to navigate. I am fascinated by this beautifully sparkly crystal. I have researched the history and have used the Sunstone in several stories. I even have one of my very own that stays on my desk. The Spar has a magical property to split light. Pretty cool, huh?
So, for all the rational people who need to understand the practical side of where the magic comes from, these details are especial for you. The Spar is a crystallized calcium carbonate. It has a unique birefringence for polarizing light. Birefringence is the marvelous - dare I say magical? - property of double refraction that splits the sunshine. If a ray of light goes through birefringent material, it is split by polarization into two rays on barely different paths.
Most light we see unpolarized, like sunlight, light bulbs, and firelight. So what is polarized? Many of us are familiar with polarized sunglasses. The glasses help filter out the glare and clear up the image of what we see. That's a basic principle of the Sunstone. An unpolarized ray of sunlight passes through the crystal and is divided into two rays of perpendicular polarization directed at different angles. This is called double refraction. It splits the ray of sunshine and makes clear rays.
At that time, most maritime navigation was done based on the sun and stars. This was fine as long as they could be seen. But, during storms or cloudy days - even weeks - a ship could drift far off course. The Vikings could use the Sunstone on cloudy days or at twilight to identify the direction of the sun within a few degrees. This is one of the reasons they were able to accomplish their amazing travel, that they were not taken off course during long periods of cloudy conditions.
See, it is just a little Icelandic magic!
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