Thorium

Thorium is a silvery, weakly radioactive metal, about three times as common in Earth's crust as uranium. Jöns Jacob Berzelius discovered it in 1829 and named it after Thor, the Norse god of thunder. It is found mainly in monazite sand, often as a by-product of mining rare earths.

Thorium decays so slowly that its half-life, about 14 billion years, is roughly the age of the universe. Thorium oxide glows brilliant white when heated, so it was used in gas lantern mantles; thorium also went into some camera lenses and tungsten welding electrodes. Thorium cannot sustain a chain reaction itself, but in a reactor it can absorb neutrons and turn into uranium-233, which can, making it a candidate nuclear fuel.