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 decided to go on strike. (AUDIO: The Reaping)

The Tenth Doctor feared that if Queen Victoria was infected by the wolf in 1879, creating the "Empire of the Wolf", that there would be "rockets and spaceships fueled by coal and powered by steam, leaving history devastated in its wake." (TV: Tooth and Claw)

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