"Lithium crystals", as specified in the Star Trek: The Original Series episode "Mudd's Women". Later retconned to "Dilithium crystals". And, they didn't actually power the Enterprise. They were placed in an "articulation frame" and the energy created from matter-antimatter annihilation focused or amplified through them (U.S.S. Enterprise Technical Manual). This process had a tendency to cause the crystals to decrystallize (Star Trek IV: The Voyage Home).