Before there was Kryptonite, there was … K-Metal. Mark Evanier explains:
As we all know, just about the only thing that can hurt Superman (besides copyright lawsuits) is the mineral known as Kryptonite. The history of this plot device is a bit convoluted and there are those who disagree about some aspects of it. But it is known that the idea began with a script that Superman co-creator Jerry Siegel wrote in 1940. In that 26 page story, Superman encountered a substance called “K-Metal,” a radioactive remnant of his home planet of Krypton, and found that it weakened him to the strength of a normal man.
The story was illustrated by Siegel’s partner Joe Shuster before the editors at DC Comics decided against publishing it. Various reasons have been cited for this decision. It was too long. They didn’t want to introduce that vulnerability into the Superman strip. And there was a scene in which Lois Lane learned Superman’s secret identity. In any case, the story was never printed. Years later, researchers came across Siegel’s original script and collectors found a few pages of Shuster’s artwork.