Coal

Coal was a black stone used on Earth for burning. (TV: "Mighty Kublai Khan") The Sixth Doctor defined it "just fossilised plant life". (TV: The Mark of the Rani)

In 1984, British coal miners. (AUDIO: The Reaping)

By the 30th century, coal was no longer used for power, and Roz Forrester did not know what it was. (PROSE: The Nuclear Option)