The sapphire stone Corundum is the scientific word for sapphire crystals. A sapphire (corundum) is an aluminum oxide mineral (Al2O3). Blue corundum is known as Blue Sapphire, pink corundum as Pink Sapphire, and so on. The color of sapphires is due to impurities that give them different colors: yellow, green, or red. Sapphires are always clear or faintly colored. Sapphires are very hard, durable, and flexible. They are used for cutting tools, such as knives, scissors, and saws. Sapphires are also used in jewelry making because of their beautiful color and sparkling appearance. Blue sapphires are more valuable than other colors of sapphires. This is because only a small amount of blue carbon is needed to create a blue color in a sapphire. Other colors require more expensive materials to achieve the same result. For example, a pink sapphire needs more almandine garnets than a blue one to produce similar results. Also, blue sapphires tend to be larger than other colors. This is because more blue carbon can be added to the crystal's structure before it becomes too large to be useful.