Kryptonite (Films)

Kryptonite is an element created from the substance of the planet Krypton after its destruction. A radioactive element, it weakens and eventually kills Kryptonians. A synthetic version was created by August "Gus" Gorman at the behest of Ross Webster, but the 0.57% of the element which could not be replicated by Earth materials were replaced- at Gorman's suggestion- with tar. The result was a Kryptonite that amplified Superman's dark side, forcing his good and evil sides to battle for dominance- a battle which the good side won.