Historians believe that Iolite helped Vikings find the hidden sun. In the cold waters navigated by the Vikings, there is a natural phenomenon called ‘Arctic sea smoke’. When cold air moves across warmer water, vapor can rise, obscuring the sun like a fog. The sky may be fairly clear, but the horizon, land, and other objects can be impossible to distinguish, making navigation tricky. By looking up at the cloudy sky through a thin piece of iolite, a Viking navigator could locate the sun and ascertained his position and direction.