Kyanite is a beautiful blue mineral with elongated crystal blades, and it is a staple in any amateur or museum quality mineral collection. Its color ranges from light to dark blue, but it can also be white, gray or — less commonly — green. Kyanite has two distinct hardnesses, depending on the direction tested. It has hardness of 5 if you scratch vertically along the crystal face and a hardness of 7 if you scratch horizontally across the crystal. Kyanite has the chemical formula Al2SiO5, which is the same as the minerals andalusite and sillimanite. These minerals are polymorphs, meaning they have the same chemical composition but different crystal systems.

As rocks are buried deep in the ground, the heat and pressure on the rocks increases, which causes internal deformation over time. While the rock is changing, new minerals can grow in place of the original ones. This process is called metamorphism, and the resulting rock type (metamorphic) is one of the three you learn about in introductory geology. Kyanite, andalusite and sillimanite each form within a specific pressure and temperature range, so geologists studying metamorphic rocks can use these aluminum silicate polymorphs as index minerals to infer the temperature and pressure that the rocks formed under.

Minerals that form at higher temperatures and pressures are known as “high grade,” and minerals that form at lower temperatures and pressures are known as “low grade.” Kyanite and andalusite are moderate grade metamorphic minerals, and sillimanite is a high to very high grade. These aluminum silicate polymorphs are often associated with schists and gneisses.

To see our excellent example of kyanite, come visit the Dice Mineralogical Museum on the first floor of North Hall, open 1:30 p.m. to 5 p.m. on Mondays, Wednesdays and Fridays.