Definition
It is frequently used as a gemstone because of its translucent variant. Iolite is a name derived from the Greek word meaning violet. Dichroite is an ancient term for cordierite that refers to the mineral's potent pleochroism and means "two-colored rock" in Greek. It has also been referred to as "water-sapphire" and "the Vikings' Compass" since the Vikings used it to determine the direction of the sun on cloudy days. This operates by figuring out which way the sky above is polarized. Even when the sun's disk is hidden by thick fog or is barely below the horizon, light dispersed by air molecules is polarized, with the direction of the polarization being at right angles to a line to the sun.
Blue iolite also goes by the name of steinheilite, which honors Fabian Steinheil, the Russian military governor of Finland who noted that it was a distinct mineral from quartz. Another form of iolite produced by heat treatment is called praseolite. Contrast it with prasiolite to avoid confusion.
Origins
Iolite of gem grade changes hue according on the light angle from sapphire blue to blue violet to yellowish gray to pale blue. Sometimes iolite is used as a less costly alternative to sapphire. It is significantly softer than sapphires and is widely distributed in Australia, Brazil, Burma, Canada, India, Madagascar, Namibia, Sri Lanka, Tanzania, and the US. The biggest known iolite crystal was found in Wyoming, United States, and it weighed more than 24,000 carats.