From Halopedia, the Halo wiki
Zenostium is a type of superconducting plasma.
Zenostium plasma is a critical component in the manufacturing of antigravity plates. The flux tube between the gas giant Hephestes and its natural satellite, Nephis, is a source of zenostium that is mined by the Nephis Coalition. Due to the importance of the flux tube to the United Nations Space Command's infrastructure, any threat of civil unrest on the moon or commercial rivalry between the Nephis Coalition and competitors is taken very seriously.