Iolite comes from the Greek « ios » meaning violet. It is also called by others names such as Cordierite from the French mineralogist Pierre Louis Antoine Cordier; another is Dichroite from the Greek word meaning « two colored », because of the strong pleochroism of iolite.
Iolite has different colors in different directions in the crystal. A cube cut from iolite will look violetish blue, almost like a sapphire from one side, colorless from the other, and a brownish yellow from the top.
This property helped Leif Eriksson and the other legendary Viking explorers that went far out into the Atlantic Ocean to determine the exact position of the sun, and thus navigate safely to the new world and back.
Iolite was sometimes called « water sapphire », which is incorrect and now prohibited.