Lava crystals were a type of crystal that could be found on the hellish world of Mustafar. The native fire rats were known to gnaw on lava crystals. While recounting a tale to the prisoner Thom Hudd about the exploits of the droid captain Spikewheel and her crew on Lotho Minor, the Sith attendant Vaneé likened the fear gnawing at the group to rats gnawing at lava crystals.
Despite most Force users wielding a blade of pure plasma powered by their kyber crystals, there was actually a far more dangerous and deadly type of lightsaber. Rather than using typical lightsaber crystals like those found on Ilum, the Lava blade used a Lava crystal found in the deep infernos of Mustafar, which was more erratic and unpredictable, causing a less focused blade.
On top of this, the crystal produced a blade of pure lava making it much heavier and capable of striking through thicker objects and opponents. This however wasn’t why it was so feared. Because of it’s lava blade, the saber would commonly gurgle out flames and chunks of molten lava in all directions making it horrifying to face for opponents. The only thing protecting the user of the blade was the force itself… so this blade was not to be used by non force users.