Iolite, and the Viking-Compass Claim

Iolite is the gem variety of cordierite, a magnesium aluminium silicate. Its defining property is strong pleochroism: the same stone looks deep violet-blue down one crystal axis and nearly colourless or pale yellow down another, so a cutter must orient the rough carefully or the finished stone shows a washed-out grey. This is the basis of the frequently repeated claim that Norse navigators used iolite as a sunstone to find the position of the sun in overcast skies by polarised light.

The claim is modern and speculative; no medieval source names iolite, the sunstone of the sagas is undescribed, and calcite spar is the material usually proposed by researchers who take the idea seriously. Iolite has also been called water sapphire from its colour. Its metaphysical associations, generally of vision and intuition, are twentieth-century and rest on that colour and on the romantic navigation story.