Iolite

When Leif Eriksson and the other legendary Viking explorers ventured far out into the Atlantic Ocean, away from any coastline that could help them determine their position, they had a secret gem weapon: 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, and navigate their way safely to the New World and back.
The property that made iolite so valuable to the Vikings 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.