Diamonds _ made of carbon atoms, are the hardest natural substance found on Earth. Formed under extremely high pressure hundreds of miles underground, they are found in very few locations around the world. Diamond is the hardest mineral. Rubies are formed of a mineral called corundum, comprised of aluminum oxide. The red color is caused by traces of chromium. Corundum also forms sapphire in many colors, which generally come from trace mixtures of iron, titanium, & chromium.