A valuable mineral used to fuel reactors on many Federation worlds, mined on Janus VI in the Original Series episode The Devil in the Dark.
Pergium is an ore on which a number of planets depend for their power supply. In the 23rd century one of the Federation's main sources is the mining colony beneath the surface of Janus VI.
In The Devil in the Dark, production there has stopped because an unseen creature is killing miners and has stolen a vital part of the colony's reactor. Kirk and Spock discover that the creature is the Horta, a silicon-based life form defending its eggs, which the miners had been destroying without realising what they were. Once peace is made, the Horta's young tunnel for the miners, making pergium and other minerals far easier to reach.
Pergium mining is mentioned again in the Deep Space Nine episode Prodigal Daughter, where it is the business of Ezri Dax's family.