Kyanite

Kyanite forms in metamorphic rocks and has is a member of the silicate family. It's chemical composition is Al₂SiO₅.

Kyanite is unique in that it has two hardnesses due to its directional crystal growth. It's a 4 on one plane and a 7 on another. It has a distinct crystal cleavage. 

Kyanite is most often colorless, however gem kyanite is a vivid and brilliant blue, used as a sapphire imitation for a very long time. Other colors are white, gray, green, orange, or black.