Viking Sunstone found in ship wreckage! Calcite crystals like Icelandic Spar create a double image, splitting light into two rays. If the crystal is held east-west, the double image becomes a single image and thus allows a sailor to locate the sun. The crystal's refractive qualities continue to be useful even in low light, on a cloudy day, or for a while after the sun has set.
Iolite. Known as the gemstone of the Vikings, Iolite was used by Norse and Viking explorers as a navigational aid, as a properly oriented stone would change color in relation to the direction of sunlight, allowing for crude navigation in the fog- enshrouded northern Atlantic waters where the direction of the sun was otherwise impossible to discern. https://www.etsy.com/listing/126884990/sterling-silver-ring-with-koi-fish