A rock of Trellium-D.
Trellium is a mineral used as insulation against anomalies by ships in the Delphic Expanse. Trellium comes in multiple forms. Trellium-A is extremely common and easy to synthesize. However, it is trellium-D that is useful as insulation, and it is rare and difficult to process.
Trellium can only be synthesized in a liquid state, though the synthesized form is just as useful as the natural ore. While still a liquid it must be immediately applied to the inner hull, where it will then set. The synthesis process is very complex and very dangerous, due to the high levels of pressure and radiation involved. (ENT: "Rajiin")
The NX-01 Enterprise visited a trellium mining planet in September 2153 to gain information from the Xindi Kessick. (ENT: "The Xindi") Enterprise later began to seek trellium to insulate its hull, but attempts by T'Pol and Commander Charles Tucker to synthesize trellium-D following protocols provided by the chemist B'Rat Ud proved fruitless after several weeks' efforts, succeeding only in destroying an Enterprise science lab. (ENT: "Rajiin") By February 2154, Enterprise had 60 kilos of trellium-D in it's storage bay. (ENT: "Damage")
trellium asteroid field
Enterprise later discovered a trellium asteroid field while following the distress signal of the Seleya, but could not use the trellium mined there because of its effects on Vulcans. Trellium is a deadly neurotoxin to Vulcans, gradually degrading their neural pathways. This has the initial effect of removing emotional inhibitions, but will eventually induce insanity and violent rages. Over time the neurological damage becomes irreversible, eventually leading to death; this was the fate of the Seleya crew. (ENT: "Impulse") It is likely that the Vaankara crew suffered a similiar fate. (ENT: "The Expanse")
T'Pol suffered severe exposure to trellium-D while in the asteroid field, and though Doctor Phlox was able to reverse the neurological damage, she continued to hunger for the release from the constraints of logic the trellium had allowed. She developed a method of injecting it into her body and became addicted to it. With Phlox's help, she successfully withdrew from the substance, but her ability to control her emotions was permanently damaged. (ENT: "Damage")
Because of the deadly effect lining the ship's hull would have on T'Pol, the Enterprise crew was forced to place the trellium-D mined in the asteroid field in a biohazard locker. (ENT: "Impulse") In order to take measurements of Sphere 2, Tucker lined the hull of Shuttlepod 1 with trellium to avoid anomalies. (ENT: "Exile")