The BBC article mentioned refraction, but Iceland Spar makes it possible to see the direction of polarization of skylight, which means it could be used on small

The BBC article mentioned refraction, but Iceland Spar makes it possible to see the direction of polarization of skylight, which means it could be used on small, clear patches of sky on an otherwise overcast day to find the direction to the sun. This is another way it could act as a "sunstone" for navigation. Whether it actually was used this way is unfortunately not known.