Q: If superman were from earth, what would be the equivalent of kryptonite?
A: I believe the point of the question is: What Element was Kryptonite, when Krypton was intact, before it was iradiated to its Lethal state by the death of Krypton and its journey through space? I would go with Lead. its common and versitile on earth, and one could argue the because it originated as lead, that that is why Lead best shields its effects, and earthly lead is capable of blocking the X-ray vision. true X-rays as we know them in medical use require a special plate behind the subject to function, and are harmful to people, which last i checked Superman's X-ray vision isn't, or who knows how many people he would have eradiated by now, by constantly scanning them.