The name iolite comes from the Greek word "ios," which means violet. Iolite is sometimes known as "water sapphire" because of its light violet blue color, but other iolite gemstones may range in color from clear to honey yellow to deep violet or indigo blue.
The ability of iolite to exhibit different colors depending on how it is cut is what led Viking explorers to use it for navigation - as a polarizing lens to look directly at the sun.
Composed of hydrous magnesium and aluminium silicate, iolite has a hardness that resembles quartz. Its Mohs hardness rating is 7 to 7.5.