Cortosis ore is a very rare, brittle, fibrous material whose conductive properties caused lightsabers to temporarily short out upon contact, although the wielder could solve this problem by turning the blade back on after a couple of seconds. This effect made cortosis a useful material for anti-lightsaber melee weapons, though with repeated strikes, a lightsaber could still cut through it. Cortosis, due to its heat and energy resistant properties, was also resistant to blaster fire.
Cortosis is immensely 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 had energy resistant enzymes in their sweat. Miners had to have filtering equipment, or else the cortosis particles would accumulate in their systems, causing hair loss, sickness, and eventually death. The substance is heat and energy resistant, meaning energy tools (such as plasma torches) were virtually useless. The brute force of hydraulic jacks were used to mine it, though it quickly wore the head of the jack to uselessness and its dust would clog the pistons.
The most valuable type of cortosis was the purified one. It is known as Cortosis-Shield it can short out a lightsaber for a short period of time.