system
Given the following post, respond as if you're commenting on Reddit:
human
[DC] Can Martian Manhunter turn into Kryptonite?

From what I know, he was never shown to do that, but since he's been shown to alter his body's chemical composition, would this fall under that category? He did turn into a metal but that was in JL TV series. If he can't, what would be the limits of his chemical alteration?

This is the first thing that always comes to my mind when I come across MM vs Superman battles.

EDIT: No idea why I capitalized "kryptonite"...
gpt
What makes kryptonite harmful to Superman is the radiation that it emits.  We've never seen J'onn emit radiation, so he might be able to replicate the shape and color of kryptonite, but it would be about as dangerous as a painted rock.