Cortosis was the marvelous metal that could magically deactivate lightsabers. It was mined on a handful of planets like Darth Bane's homeworld of Apatros. You could make swords out of it, or shadow trooper armor, or giant evolution-speeding monoliths.
Lightsabers could cut through anything except cortosis (with the exception of one being wielded by The Divine Beard). And electrostaffs. But electrostaffs were different because it was the electrical energy that blocked the lightsaber. Cortosis was just a metal that they couldn't cut through. Oh, and lightsabers couldn't quite cut through Yuuzhan Vong armor either, because, um... never mind. The point is lightsabers could never, ever cut through cortosis, ever. Which means that, if George Lucas ever wrote a script with cortosis in it, the first thing that would happen is a lightsaber would cut through it.
In the days of Knights of the Old Republic, any old sword could block a lightsaber, no matter what it was made of. Stupid old-fashioned lightsabers.