Ruby is a variety of Corundum (Aluminum Oxide) and so is Sapphire. Well, to get this strait, (people usually get confused when I tell them about this) we have the mineral Corundum. Now some of those Corundum crystals are growing in Iron and Titanium deposits, causing, over time, the Iron and Titanium Atoms to get into the crystal, therefore turning it blue! Now some other Crystals are growing in deposits of Iron and Chromium, turning them red! So the only difference between Ruby and Sapphire is the color. So Ruby is red Corundum and Sapphire is Corundum of any other color.