When Viking explorers ventured far out into the ocean, they had a secret gem weapon to aid navigation: Iolite. The Viking mariners used thin pieces of it as the world's first polarising filter. Looking through an iolite lens, they were able to determine the exact position of the sun, even through cloud cover and navigate their way safely to the Vineland and back.
The property that made iolite so useful to ancient mariners is its extreme pleochroism. Iolite has different colours in different directions in the crystal. A cube cut from iolite will look a more or less violet blue, almost like sapphire, from one side, clear as water from the other, and a honey yellow from on top. In the past, this property led some people to call iolite 'water sapphire', though the name is now obsolete.
Pleochroism may have been helpful in navigation but it certainly makes life difficult for the cutter. If iolite is not cut from exactly the right direction, no matter what the shape of the raw crystal, its colour will not be shown to its best advantage.