Iolite is also called cordierite after the French Geologist Pierre Cordier. Light is polarised through iolite, a feature employed by the Vikings to locate the sun through clouds on overcast days. Viewing the sky through an iolite crystal at sea revealed the suns exact position, assisting daytime navigation.
Iolite is a blue to violet stone showing highly saturated colours through the crown of stones that were correctly oriented when cut. Yet looking through the pavilion may show a colourless or yellowish stone. This is due to a strong trichroism, where three colours or shades of colour are visible when viewing the stone from different directions. This is the reason why iolite is also called water sapphire (a misleading description in the gem trade).
Source locations for Iolite include: Brazil, Myanmar, India, Sri Lanka and Africa.
Iolite is not normally treated.