- "The Hardening Charm will turn an object into solid stone. This can be surprisingly handy in a tight spot. Of course, most students only seem to use this spell to sabotage their fellow students' schoolbags or to turn a pumpkin pasty to stone just before someone bites into it. It is unwise to try this unworthy trick on any of your teachers."
- —Miranda Goshawk, Book of Spells[src]
Duro is Latin for "I harden" or "I stiffen"; the same term is Portuguese for "hard" or "rough"; it makes sense given J. K. Rowling's relation to the language.
- Harry Potter and the Deathly Hallows (First appearance)
- Wonderbook: Book of Spells (First identified as Hardening Charm)