Medieval Norse sailors may have used a unique navigation tool called solarstein to guide their voyages. Crystals of Iceland spar could detect polarized sunlight, helping seafarers determine the sun's position even under fog or clouds. Modern tests show these crystals can pinpoint the sun with high accuracy even after sunset. Although no sunstones have been found in Viking graves, one was discovered in a shipwreck dating from the sixteenth century. Early Icelandic texts from the 1200s mention the