Which fictional material appears in Superman stories?

Kryptonite is a fictional material that appears primarily in Superman stories. In its most well-known form, it is a green, crystalline material originating from Superman's homeworld of Krypton, that emits peculiar radiation that weakens Superman, but is generally harmless to humans when exposed to it in short term. There are other varieties of kryptonite such as red and gold kryptonite which have different but still generally negative effects on Superman. Due to Superman's popularity kryptonite has become a byword for an extraordinary exploitable weakness, synonymous with "Achilles' heel".