Corundum: ruby, sapphire, and the second hardest natural mineral

Ruby and sapphire are both corundum, sitting at 9 on the Mohs scale, second only to diamond among natural minerals. Red corundum is called ruby, every other color is called sapphire, including the blue variety most people picture as well as pink, yellow, and colorless material. The overwhelming majority of ruby and sapphire on the market, at essentially every price point, has been heat treated to improve color and clarity, a practice so standard in the trade that untreated stones are specifically marketed as such and carry a real premium. Because of the extreme hardness, natural ruby and sapphire survive stream transport extremely well and can turn up as water worn pebbles in placer gravels in a handful of well documented regions around the world.