Kryptonite is a fictional material that appears primarily in Superman stories. In its best-known form, it is a green, crystalline material originating from Superman's home world of Krypton that emits a peculiar radiation that weakens Superman, but is generally harmless to humans when exposed to it in the short term but deadly in the long 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". Batman and Lex Luthor are two notable individuals who have pieces of kryptonite with them, with the former being trusted by Superman himself to stop the latter in case he goes rogue or does something out of the ordinary and the latter to ward off Superman's presence and at times incorporating it into weapons to try to kill him.
