Carbon
From Tardis Wiki, the free Doctor Who reference
Carbon is one of the most common elements in the universe. It is highly abundant in the Earth's crust, and was part of the reason why the Urbankans came to Earth. It was one of the minerals they planned to take. (DW: Four to Doomsday)
An atom of carbon was used as the world for the miniaturised people from the Composition Adjustment Teleport. (DWA: Inter-Galactic Cat)
Most life-forms in the universe were carbon-based, although others were known to be based on other elements, such as silicon and calcium.
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