Radium

Radium is a highly-radioactive material well-known to the modern science of Earth. However, in the late 19th century, the properties of radium are mostly unknown. It appears in the Martian Dreams.

Description
Since it was used by the Martians as an energy source for both their machines and weapons (such as the freeze and heat ray guns), a supply of radium was required by the Avatar to make use of Martian technology. The high levels of radioactivity in radium make it poisonous and potentially lethal -- a fact proved by the fate of Marie Curie, who likely died of leukemia (post-Martian Dreams) because of her careless handling of the substance. To avoid a similar demise, 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 make their dwellings right next to the useful radium veins in the mines, making mining it a dangerous undertaking.