Iolite is the gem variety of the mineral cordierite. The name "iolite" comes from the Greek word "los" meaning violet. Another old name is dichroite, a Greek word meaning "two-colored rock", a reference to cordierite's strong pleochroism. It has also been called "water-sapphire," because its color is akin to blue sapphires, and "Vikings' Compass." When Viking mariners sailed the ocean, they used thin pieces of iolite as a polarizing filter. Looking through an iolite lens, they could determine the exact position of the sun on overcast days and navigate safely.
Pleochroism is very pronounced in iolite and is seen as different color shades in the same stone. In the viewing an iolite stone, the colors violet blue, yellow gray, a light blue, and even a green brown can be seen, all a result of pleochroism.
Pleochroism is caused by the absorption of different wavelengths of light traveling through different directions in the crystal. If in one direction, all wavelengths but yellow and blue are absorbed, then the crystal will be green (yellow and blue make green). If in another direction, all wavelengths are absorbed but blue, then the crystal will appear blue. If the crystal is turned from the first direction to the other, then it will change its color from blue to green.The major sources of gem grade iolite are Sri Lanka, India, Madagascar and Burma. Iolite's beauty is complex, and yet the stone remains very affordable.