Sapphire and ruby are both aluminum oxide - i.e. 'alumina' - just with different kinds and amounts of impurities. Yes, it is technically the stuff you get when

Sapphire and ruby are both aluminum oxide - i.e. 'alumina' - just with different kinds and amounts of impurities. Yes, it is technically the stuff you get when you throw aluminum powder over a fire and it lights up and burns brightly. But the crystals you obtain that way are very tiny.