*kreus-

Proto-Indo-European root meaning "to begin to freeze, form a crust."

It forms all or part of: crouton; crust; Crustacea; crustacean; cryo-; cryogenic; crystal; crystalline; crystallize; custard; encrust; Kristallnacht.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit krud- "make hard, thicken;" Avestan xruzdra- "hard;" Greek krystallos "ice, crystal," kryos "icy cold, frost;" Latin crusta "rind, crust, shell, bark;" Lettish kruwesis "frozen mud;" Old High German hrosa "ice, crust;" Old English hruse "earth;" Old Norse hroðr "scurf."