Radium is an element, a highly radioactive metal. Once commonly used to paint glow-in-the-dark watch faces, radium is now known to be extremely toxic.
Radium, which was discovered in 1898 by the scientist Marie Curie and her husband Pierre, is one of the rare metals that's radioactive. It's the heaviest element of its type, the alkaline-earth metals, and it is contained within uranium ore. In addition to watches and clocks, radium was used in the past to treat cancer. Today its main use is industrial, in giant x-ray devices that scan for flaws in metal machines.