Kryptonite was a fictional crystaline substance that weakened Superman. Colloquially, it was a synonym for a person's weakness.
The white-ish, semitransparent, glass-like material that contained Kryptosa's journal reminded Lucretia Scannon and Ash Juliandis of Kryptonite. (PROSE: Down)
Mike Yates asked the Fourth Doctor if the meteor was weakening him like Superman and Kryptonite. (AUDIO: Starfall)
Rose Tyler discovered that a Witiku was weak to her hot cup of Jinnera and compared them to Superman and Kryptonite. (PROSE: The Price of Paradise)
Gwen Cooper knew H&M was like Kryptonite to Rhys Williams. (PROSE: The House That Jack Built)