In the fifth episode of "Star Wars: The Acolyte," Jedi lightsabers hit the gauntlets (or helmets) of the alleged Sith, and their blades short out for a moment.
Cortosis is dense and conductive in such a way that it can absorb blaster bolts and feed energy back into a lightsaber just enough to temporarily short it out.
Some descriptions refer to Cortosis as fibrous and can be woven into other metals like the armor of this alleged Sith, while others refer to the rare metal as brittle.