Trellium is a mineral used as insulation against anomalies by ships in the Delphic Expanse. The NX-01 Enterprise visited a trellium mine in 2153 to gain information from the Xindi Kessick. (ENT: "The Xindi")
Trellium affects Vulcans greatly, in that Trellium gradually degrades their neural pathways. This has the initial effect of removing emotional inhibitions, but will eventually induce insanity and violent rages. (ENT: "Impulse") T'Pol became addicted to trellium during her time in the Delphic Expanse. (ENT: "Damage")