Picture a Viking warrior at the head of a longship, navigating by consulting a small transparent crystal. That ... doesn't seem possible, right? Crystals aren't exactly GPS. Yet that's exactly what the Vikings did in major legends such as "The Saga of King Olaf." There isn't any definitive proof, but there's a lot of circumstantial evidence to suggest those myths have roots in reality. In the 10th century, the magnetic compass hadn't yet come to Europe. That meant the only way to travel by sea was by the sun or by the stars. And since the skies of Scandinavia are notoriously cloudy, getting around that way would be a tall order.

Unless, of course, you had some way to see where the sun was, even behind a cloud. Meet Iceland spar, a transparent variety of calcite found in large deposits on the volcanic island. These crystals are special for a lot of reasons. They cleave easily from the stone and they're a piece of cake to break into small, handheld shapes. Oh, and they've got beautiful birefringence. That means they split light into two parts — look at one sock through the crystal, for instance, and you'll see a complete pair.

But what really would make the crystals helpful for navigation is the way they interact with the polarization of the light. Polarization essentially refers to the angle at which light strikes a surface. When you rotate a piece of Iceland spar you'll eventually hit the angle at which both images are equally bright. Then, you can start scanning the sky for the sun, even under a heavy cloud cover. Because some of the sunlight will come through the clouds, the polarized crystals capture a pair of concentric rings of light around the sun. Voila — you've got yourself a sun detector good enough to launch a hundred screaming berserkers.

Besides the fact that they would have worked the way the sagas said they did, there are some pretty compelling reasons to think Iceland spar is, in fact, the stone of legend. In 2013, a French team discovered the wreck of the Alderney, complete with a large chunk of calcite. The Alderney was an Elizabethan ship that sank in the year 1592, more than 500 years after King Olaf. But the fact that the stone was discovered so close to known navigational equipment could suggest that it stayed in the sailor's toolkit long after the age of Vikings had passed.

Perhaps most excitingly, a new study released in 2018 suggested that the sunstone could easily explain the range of the Viking sea empire. The computer model simulated 3,600 voyages in the season of heavy ocean traffic, between the spring equinox and the summer solstice. These simulated voyages also varied by cloud cover, type of crystal, and how frequently the sailors consulted them. As it turned out, using sunstones more frequently made a huge difference. With consultations scheduled every five to six hours, the boats got lost over and over again. Checking the sunstone every four hours led them safely to shore between 32 and 59 percent of the time. But if they checked every three hours instead, they found their way home during between 92 and 100 percent of voyages.

We don't have an answer to the mystery of the sunstones quite yet, but we're feeling pretty confident that Iceland spar is the missing piece. Either that, or they were following Thor the whole time.

Want to find out more about history's most metal civilization? Check out Else Roesdahl's "The Vikings" for a rollicking, historically accurate look at the sea-ravagers. We handpick reading recommendations we think you may like. If you choose to make a purchase through that link, Curiosity will get a share of the sale.