From The Stargate Omnipedia


An incredibly strong and lightweight alloy. Trinium is rather rare and not found on Earth, though the S.G.C. has secured its own off-world trinium mine. Unrefined it is extremely brittle, and unsuitable for manufacturing purposes. When refined, however, the metal is approximately 100 times lighter and stronger than steel.


Spirits - SG-1 finds a world rich in trinium, but is prevented from mining there by powerful aliens who protect the local Salish Indian population.
A Matter of Time - After a black hole destroys the iris protecting the Stargate, the barrier is replaced with a new, trinium-reinforced iris.
Between Two Fires - The Tollan suggest trading their advanced weapon technology with Earth for quantities of trinium, which is required for any device housing their phase technology. Travell notes that Earth has its own trinium mine.
Prometheus - Carter reveals that the walls of the Prometheus are enhanced with trinium alloys.