The Thousand-Year-Old Viking Sunstone

A thousand years ago, Vikings navigated with a sunstone, which they used to locate the sun on cloudy days. The stone—a calcite crystal called Iceland spar—funnels light into two beams. When the beams appear equally bright, the rock is facing the light, even if it’s obscured. Researchers now use calcite to funnel light around tiny objects for “invisibility” cloaks.