'Crouton' and 'crystal' both ultimately stem from an ancient root that was something like kreus and meant 'freezing over' or 'crusting'. Crystals were thought in Ancient Greece (not unreasonably) to resemble frost, kryos, and from that was derived krystallos.
Related words are crust, crustacean, a whole series of modern words coined from cryo-, and, surprisingly, custard.
Why is a crouton like a crystal?
https://commons.wikimedia.org/wiki/File:Crouton_skibidis_next_to_an_ice_cube.jpg
'Crouton' and 'crystal' both ultimately stem from an ancient root that was something like kreus and meant 'freezing over' or 'crusting'. Crystals were thought in Ancient Greece (not unreasonably) to resemble frost, kryos, and from that was derived krystallos.
Related words are crust, crustacean, a whole series of modern words coined from cryo-, and, surprisingly, custard.