A new form of kryptonite is introduced in Adventure Comics #279 (Oct 1960): Superboy is yet again thrust into the distant future and must prove he wasn’t just a myth, that he actually existed in the past. He finally convinces folks when he uses a piece of white kryptonite from a museum to kill all creeping plants threatening the world with its deadly rays.
I can tell we are at another point in history where ideas seem to be running thin in some areas of the DC bullpen. Repeated time travel stories, uninspired villains, new forms of kryptonite, super-cats causing permanent brain damage. We’ve seen it all before, folks!