Obi-wan had a chest that was locked to Luke's fingerprints, knowing he would be back eventually. Inside was a book on basic force techniques, a unreadable datachips, a book on lightsaber construction, and some other crap.
Luke used some sand from outside and Obi-wan's fusion furnace and made himself a crystal, whick he cut himself. This took place during "Shadows of the Empire", between Ep V and Ep VI.