Both sapphires and rubies. Plenty hard for a knife also, but not sure about the toughness. Actually with the laser industry figuring out how to make synthetic rubies with virtually no flaws, a piece big enough for a small blade would probably cost little more than many of the more exotic steels and other alloys. Ruby slippers would still be rather costly for all but the most petite footed, wonder what it would take to make one of those neat little keyring blades or something? Of course it would probably be grounds for concealed weapon or some such charge in some jurisdictions since it would be a non-metalic blade and rather hard to detect...