Kyanite, aluminum silicate, is chemically identical to two other minerals that have different crystal structures, andalusite and sillimanite. Kyanite is generally blue but can also be green or nearly colorless. It typically forms in metamorphic rocks, ones that have been intensely squeezed by tectonic forces. Kyanite is unusual in that its crystals display a different hardness in each of three directions. The kyanite from North Carolina is especially attractive and occurs in large crystal aggregates.