Ruby has been the world's most valued gemstone for thousands of years. In the ancient Sanskrit language, it was called ratnaraj or "king of precious stones". Even today, rubies are considered more valuable and rare than even the top quality colorless diamonds.
These gemstones are red variations of the mineral corundum, a crystalline form of aluminum oxide and one of the most durable minerals that exists; only diamonds are harder. The rich red color arises from the substitution of a small number of aluminum atoms by chromium atoms. Pure corundum is a colorless, trigonal crystal, but it occurs in a wide variety of colors due to infiltrations of other elements. All color variations of corundum, with the exception of ruby, are called sapphires.