Trellium is a mineral found in the Gamma Quadrant and the Delphic Expanse. In the Expanse, it is used by starships as insulation against anomalies. Trellium comes in multiple forms and 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 episode: "Rajiin")
In 2372, Quark opened negotiations with the Aulep, hoping to buy trellium from the Gamma Quadrant. The trellium was later stolen and the Aulep came looking for it at Deep Space 9. (DS9 novel: Trial by Error)