Meaning

verb cause to take on a definite and clear shape

verb make free from confusion or ambiguity; make clear

verb cause to form crystals or assume crystalline form

originFrom crystal + -ize. The double l reflects the classical stem found in its etyma, Latin crystallus and Ancient Greek κρύσταλλος (krústallos), rather than the simplified spelling of the English base noun. Compare Hellenistic Greek κρυσταλλίζειν (krustallízein, “to shine like crystal”), post‐classical Latin crystallizare (“to change into crystal”), and French cristalliser.

Etymology from Wiktionary, CC BY-SA 3.0