Firkrann crystals were a comparatively heavy lightsaber crystal mined on the planet Rafa V. The crystal imparts an electrical charge to the weapon that contains it making the saber especially devastating to droids. They were noted to be used specifically by the Dark Lords of the Sith.
When Revan entered the tomb of the Sith Lord Tulak Hord, he found a Firkrann crystal that may have been used by Hord in his lightsaber.
This crystal is used in Star Wars: The Force Unleashed as a lightsaber crystal that sometimes infuses the lightsaber with Force lightning.