Kyanite is such a beautiful stone. The best material is a clear, royal blue, good enough to give sapphire a run for its money, with a fantastic vitreous lustre. Lower grade material can be streaky and cracked, which makes it rather brittle, so it's safest to stick with the good stuff! I have high grade blue material, nice and clean, and have a look; it would make an excellent sapphire substitute. Kyanite is also strongly trichroic and you can see on some of the photographs the different colours; cobalt, violet and clear.