Carbon

Carbon was one of the most common elements in the universe. It was highly abundant in the Earth's crust, and was one of the reasons the Urbankans came to Earth. It was one of the minerals they planned to take. (TV: Four to Doomsday).

A group of Slitheen came to Earth to crush the planet into a diamond, using the carbon in the atmosphere. (TV: The Gift) Baltazar attempted to use plasma fire to compress all carbon-based lifeforms on Earth into diamonds. (TV: The Infinite Quest)

An atom of carbon was used as the world for the miniaturised people from the Composition Adjustment Teleport. These people were divided into the Blue and Green armies, who warred with each other. (PROSE: Inter-Galactic Cat)

Humans were carbon-based, (TV: The Hand of Fear) as were most other life-forms in the universe, although some were known to be based on other elements, such as silicon and calcium. (PROSE: Eternity Weeps, TV: The Stones of Blood, World War Three)