Cordierite was discovered in 1813 and named after the famous French geologist Louis Cordier. As the transparent variety iolite, it is often used as a gemstone. The name “iolite” comes from the Greek word for ‘violet’.

Another old name is dichroite, a Greek word meaning “two-colored rock”. It refers to the strong Pleochroism exhibited by Cordierite. Pleochroism is an optical phenomenon in which a substance appears to be of different colors when viewed from different angles, especially with polarized light.

Cordierite has also been called “water-sapphire” and “Vikings’ Compass” because of its usefulness in determining the direction of the sun on overcast days. The Vikings having used it for this purpose.

This works by determining the direction of polarization of the sky overhead. Light scattered by air molecules is polarized, and the direction of the polarization is at right angles to a line to the sun. So even when the sun’s disc is obscured by dense fog or lies just below the horizon this can be used.

Gem quality iolite varies in color from sapphire blue to blue violet and from yellowish gray to light blue as the light angle changes. Iolite is sometimes used as an inexpensive substitute for sapphire. It is much softer than sapphires.

Iolite is abundant in Australia (Northern Territory), Brazil, Burma, Canada (Yellowknife area of the Northwest Territories), India, Madagascar, Namibia, Sri Lanka, Tanzania and the United States (Connecticut).

The largest iolite crystal found weighed more than 24,000 carats, and was discovered in Wyoming, USA.