io9 writes The secret of ancient Viking navigation was transparent crystals. This is pretty cool.
"Researchers at the University of Rennes in France have put together all the experimental and theoretical evidence they could muster, and they're pretty sure they know exactly what this sunstone was. It was, in fact, a transparent calcite crystal known as Iceland spar. It's found all over its namesake country, and Vikings could have used it to depolarize light, which means the crystal is able to split light along different axes.
How is that useful for navigation? Well, Viking sailors simply had to place a dot on the top of the crystal and then look up at it from below. The incoming light would hit the dot and seemingly duplicate it. That optical effect, amazingly enough, was all ancient navigators needed to locate the Sun, even when it was completely hidden from view."
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