It is a mineral that forms rocks. It is likewise a naturally transparent material, but the presence of transition metal impurities in its crystalline structure causes it to change color. The two most common gem kinds of corundum are ruby and sapphire. Sapphire is the natural form of quartz, which is used for tableware and windows. Ruby is a red carbonate mineral with a hardness of 7 on the Mohs scale. It is derived from the Arabic word for blood, which is appropriate because it usually contains some amount of iron oxide. Ruby can be either pure or mixed with other minerals. It is commonly found in Brazil, India, Russia, and USA. Saigons are one of the most important deposits of ruby in the world. The name comes from the Sanskrit word for wishful thinking, which is relevant since sapphires are thought by some people to bring good luck. Ruby is a hard material; this means that it is relatively resistant to physical damage such as cutting and grinding. However, due to its sensitivity to heat and humidity, it should not be exposed to direct sunlight or kept in a humid place. People have been collecting and wearing ruby jewelry for many centuries. In ancient Greece and Rome, for example, ruby was one of the six gems recommended for use in crowns.