刚玉, also known as corundum, is a naturally occurring mineral composed of aluminum oxide. It is renowned for its hardness, ranking just below diamond on the Mohs scale, making it an important material in various industrial applications, including abrasives and cutting tools. In its pure form, 刚玉 is colorless, but it can appear in various colors due to impurities. The most famous varieties are ruby, which is red due to chromium, and sapphire, which can be blue or other colors depending on the presence of different elements.