Group: Silicates
Species: cordierite
Hardness: 7 – 7.5 (Mohs scale)
Chemical composition: Mg2Al4Si5O18
Colors: Blue, violet, brownish
Hardness: 7-7.5
Formation: Orthorhombic, short prisms
Principal Sources: Brazil, India, Madagascar, Myanmar, Sri Lanka and the USA
Special Notes: Iolite is the gem version (translucent) of cordierite. Iolite is typically blue, violet or brownish. The best color is a lovely violet blue that is unlike other gemstones. Iolite has different colors in different directions in the crystal. A cube cut from iolite will look a violetish blue almost like sapphire from one side, clear as water from the other, and a honey yellow from the top. This property led some people to call iolite “water sapphire” in the past, a name that is now obsolete.
Pleochroism is very pronounced in iolite and is seen as three different color shades in the same stone. In the viewing an iolite stone, the colors violet blue, yellow gray and a light blue can be seen.
Pleochroism makes things difficult for a gem cutter. If iolite is not cut from exactly the right direction, no matter the shape of the rough, its color will not show to its best advantage. Iolite is frequently step cut to enhance color and sometimes windowed or shallow cut to lighten tone. The cutter must orient the rough carefully, taking iolite’s trichroism of blue, gray and near colorless into account.
The name iolite comes from the Greek ios, which means violet. The Vikings probably mined iolite from deposits in Norway and Greenland. Viking sailors allegedly used iolite as a polarizing filter to find the sun on cloudy days for a safe offshore navigation.
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 position, they had a secret gem weapon: iolite. The Viking sailors allegedly used thin pieces of iolite as the world’s first polarizing filter. Looking through an iolite lens, they could determine the exact position of the sun on cloudy days, and navigate safely to their new worlds and back.