Sapphires and rubies look quite different. While both precious stones, sapphires are known to be deep blue and rubies are red. However, they are actually the same stone. Corundum is a crystalline form of aluminum oxide. It is naturally clear but can be different colors when it has impurities.
If corundum is red, it is known as ruby. If it is pink-orange, it’s called padparadscha. All other colors are referred to as sapphire. So sapphires can actually be other colors besides blue. For example, there are green sapphires.
Apart from its beauty, corundum is known for it's hardness (it can scratch almost every other mineral) and it's high density. It is mined in Russia, Zimbabwe, Sri Lanka and India.