Kyanite

When they get metamorphosed, high-aluminum rocks, like your typical shale and slate, tend to grow Al₂SiO₅. That may form one of three minerals, or polymorphs, depending on the temperature and pressure. Kyanite is the high-pressure, low-temperature form. This rock-shop specimen is cleaner and bluer than your typical occurrence, but not by much. See it and learn more in the Mineral Picture Gallery.
Kyanite — Geology Guide photo