Radium

Radium is a highly-radioactive material well known to the science of Earth. However, that not always was the case. In the late 19th century of Martian Dreams, the properties of Radium are mostly unknown.

The Avatar needs Radium however, since it was used by the Martians as an energy source for their machines and weapons. The high radioactivity of it is poisonous and can be lethal, as the later fate of Marie Curie can attest, who died early because of her careless handling of the stuff. To avoid this, the Avatar had to carry Radium in lead boxes and manipulate it only with the help of tongs.

Another problem is, that on Mars, Rockworms love it to dwell right next to the useful Radium veins in the mines, making mining it a dangerous undertaking.