A crystal's hardness never is the same all over; it varies according to the different facets. These variations are usually small, of about 0.5 degrees in Mohs' scale, and that is the reason that diamonds can be cut with their own powder. Kyanite is also known as disthene, as variations in hardness can be quite extreme (the most extreme in the mineral world) oscillating between a minimum of 4.5 and a maximum of 7. Its pearly sheen and attractive colouring, which typically is blue, but can also be green, black or golden, make specimens of good quality obtain a considerable price.