Interesting that you should bring up the Powerstone story.
It's actually a two-parter - the first installement was published in
Action Comics #47 (1942). If you want to read it, it was reprinted in Les Daniels' book,
Superman: The Complete History. I don't remember what issue number contained part two.
The "Powerstone" was a stone from another planet with scientfic properties that gave Luthor powers as great as, or greater than, those of Superman. He was also able to use it to immobilize Superman.
Many speculate that this story was originally the intended sequel to "K-Metal" before it was re-written to make Professor Winton's K-Metal fragment into a slightly-different Powerstone.