In the case of the Vikings, they possibly used one of two crystals, cordierite, also called lolite, of Norway, or calcite of Iceland.
By holding a polarizing crystal up to the sky and rotating it, you can determine the direction of the Sun. Summer, when sailors far north ventured upon the seas, they were confronted with nearly perpetual daylight, preventing them from using the stars to navigate. Light consists of electromagnetic waves that oscillate perpendicular to the direction of the light's travel. When the oscillations all point in the same direction, the light is polarized.