Ruby is a precious gemstone variety of corundum (Al₂O₃), known for its vibrant red color, which is caused by trace amounts of chromium. It is one of the hardest gemstones, with a Mohs hardness of 9, making it extremely durable and suitable for use in fine jewelry. Rubies can range in color from pinkish-red to deep crimson, with the most prized stones being those with a rich, pure red hue known as pigeon blood red. Major sources of rubies include Myanmar (Burma), Thailand, Mozambique, and Cambodia. Ruby is one of the Big Four gemstones, alongside sapphire, emerald, and diamond.