Kyanite is a polymorph with two other minerals; andalusite and sillimanite. A polymorph is a mineral that shares the same chemistry but a different crystal structure with another, or other, minerals. Kyanite is an attractive mineral that has a near sapphire like blue color in some especially nice specimens. Kyanite has a unique characteristic in that it has a wide variation in hardness, in the same crystal. The hardness of kyanite is approximately 4.5 when scratched parallel to the long axis of the crystal and approximately 6.5 when scratched perpendicular to or across the long axis. Other minerals usually have variable hardness on different crystal faces due to a different concentration and orientation of the atoms in the structure. Diamond is one such mineral, a fact gem cutters take advantage of often.