Cortosis

Cortosis ore was a very rare, brittle, fibrous material whose conductive properties caused lightsabers to temporarily short out upon contact. This effect made cortosis a useful material for anti-lightsaber melee weapons, though with repeated strikes, a lightsaber could still cut through it. This substance was used extensively in shadowtrooper armor, as well as the Cortosis battle droids during the Clone Wars, and the Force-using group known as the Jensaarai. The cortosis used for this particular batch of armor was smuggled out from Bespin's Cloud City. The walls of the Imperial Palace were also lined with cortosis ore.

Cortosis was expensive to mine because it was so rare and it absolutely had to be refined. Pure, unrefined cortosis ore was - for unexplained reasons - energized, killing anyone who touched it with his/her bare skin, even Hutts, who have energy resistant enzymes in their sweat.

During the New Sith Wars, the Outer Rim Oreworks Company mined cortosis on Apatros.

During the Clone Wars, the Jedi Council discovered that the CIS was mining cortosis on Bal'demnic, and dispatched a task force led by Halagad Ventor and Sha Koon to sabotage the operation.

As stated, cortosis ore is extremely brittle, and objects cast out of homogenous cortosis have short lifespans. As such, much effort has been expended in alloying cortosis with several other metals in the attempt to strengthen it. These ventures have been successful in extending the durability of cortosis blades and armor, but lose the ability to short out a lightsaber. Cortosis alloy, though, is able to repel (block, parry, shield, etc.) a lightsaber or other energy weapon, and a few of the galaxy's finest weaponsmasters possess blades of cortosis with which to parry with lightsabers.