Tellerium

Tellerium was a necessary material used to stabilize the matter-antimatter reaction in a warp core. On an starship, a small vial of tellerium was useful for an extended period of time.

The USS Voyager nearly ran out of tellerium in early 2372. In a desperate attempt to fill a vial of tellerium, Kathryn Janeway, Tuvok, B'Elanna Torres, and Neelix secretly transported to the Mokra-controlled planet and were nearly all captured by the Mokra Order. Neelix made it back to the starship with the tellerium. 

Tellerium