An extremely hard material used to armour the Earth outposts along the Romulan Neutral Zone, which the Romulan plasma weapon nonetheless reduces to a crumbling mass.
Rodinium is mentioned in the Original Series episode Balance of Terror. The Earth outposts guarding the Neutral Zone are built deep inside asteroids and protected by shields of cast rodinium.
When a Romulan bird-of-prey crosses the border and destroys several of them, Commander Hansen of Outpost 4 reports that his own shielding has been shattered by a single hit from an enormously powerful energy weapon. The outpost is destroyed moments later while the Enterprise watches.
Spock later examines a fragment recovered from the wreckage. He notes that rodinium is the hardest substance known to Federation science, then crumbles the sample in his hand, showing the bridge crew what the weapon would do to the ship.