Possibly also called the Mind's Eye (although I don't think the book mentioned the name except in its title), the Kaiburr Crystal greatly magnifies the Force powers of those in contact with it. Luke Skywalker and Darth Vader sought it in the Star Wars book Splinter of the Mind's Eye. Luke fought Vader on the planet Mimban and was able to fend off the Dark Lord (who presumably had much greater skill with the Force and the lightsaber) due to Luke's posession of a splinter of the crystal and channeling Obi-Wan Kenobi.