Kryptonite vs. Krypton — What's the Difference?

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 unique radiation that weakens Superman, but is generally harmless to humans when exposed to it in the short term but deadly in the long term.

Krypton (from Ancient Greek: κρυπτός, romanized: kryptos 'the hidden one') is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is often used with other rare gases in fluorescent lamps.

Something that presents a particular threat to one that is otherwise powerful

A colorless, largely inert gaseous element used chiefly in gas discharge lamps and fluorescent lamps. Atomic number 36; atomic weight 83.80; melting point -157.36°C; boiling point -153.34°C; density 3.733 grams per liter (0°C). See Periodic Table.

(figuratively) The one weakness of something or someone that is otherwise invulnerable, an Achilles heel.

The chemical element (symbol Kr) with an atomic number of 36. It is a colourless, odourless noble gas that only reacts with fluorine. It is one of the rarest gases in the Earth's atmosphere.

(countable) An atom of this element.

An inert gaseous element of the argon (noble gas) group, of atomic number 36, occurring in air to the extent of about one volume in a million. It was discovered by Ramsay and Travers in 1898. Boiling point, -152.3° C.; melting point, -156.6° C.; symbol, Kr; atomic weight, 83.8.

A colorless element that is one of the six inert gasses; occurs in trace amounts in air