"Food is one of the first of five Principal Exceptions to Gamp’s Law of Elemental Transfigur— ... It’s impossible to make good food out of nothing! You can Summon it if you know where it is, you can transform it, you can increase the quantity if you’ve already got some—”
-- Hermione Granger to Ron Weasley (DH15).
Gamp’s Law of Elemental Transfiguration is a fundamental property of magic which identifies what can be conjured and what cannot. Generally speaking, this axiom states that one cannot create something from nothing.
- There are five exceptions to Gamp's Law (DH15).
- One of the exceptions is food, which cannot be "made from thin air," but has to be summoned from elsewhere, as when McGonagal conjured a neverending platter of sandwiches to Snape's office from the Hogwarts kitchens (CS5); increased, as Hagrid did by using the Engorgement Charm on pumpkins (CS7); or transformed as Molly Weasley did when cooking (OP5). This also explains why the Room of Requirement would not provide food to Dumbledore's Army, so they had to get meals from Aberforth at the Hog's Head (DH29).
- Water, however, is not an exception because it can be transformed out of nothing with the Water-Making Spell (Aguamenti), as Harry did when filling the water goblet for Dumbledore in the sea cave, and using water to put out the fire on Hagrid's Hut (HBP26, HBP28).
Possibly named for Ulick Gamp, Head of the Wizengamot before being elected the first Minister for Magic from 1707-1718. During his time in office, the Wizarding World first imposed the International Statute of Secrecy.
We can take a guess at a few of the other exceptions to Gamp's law. These are just suggestions and educated speculations:
- Wands cannot be conjured by a witch or wizard, but must be manufactured by a true wand maker, such as Mr. Ollivander or Gregorovitch. The reason for this is probably that wands themselves are "sentient" or self-aware, and they choose the wizard, not the other way around (Pm). The Weasley Twins could only manage "fake" wands (GF4), and broken wands can be dangerous and must be replaced (PA4). Or a wand can change allegiance, as the Elder Wand did between Draco and Harry (DH35). However the only time a wand was repaired by a wizard instead of a wandmaker was when Harry using the Elder Wand to fix his holly-phoenix wand after he became Master of Death. That was a very special case, and he used a very special wand (DH36).
- Real money cannot be conjured, which is why there can still be poverty in the Wizarding World. Harry and Draco inherited family money, but the Weasleys were not as fortunate; Ron is plagued with hand-me-downs from his first wand to his pet Scabbers, not to mention his homespun clothing (PS6). Harry gave the Weasley Twins the gold he won for the Tri-Wizard Tournament after Ludo Bagman swindled them with fake Leprechaun gold that disappeared (GF37). When Merope Riddle's marriage fell apart, her clothing fell to rags and she had to sell Slytherin's Locket to Caractacus Burke for a mere 10 galleons (HBP13). Harry was shocked that Merope seemed to choose death instead of increasing food to keep herself and her child healthy.
- Potion ingredients are never conjured, and must be grown, gathered, or purchased, often at a hefty price, which is why Slughorn was happy to collect unicorn hair and Acromantula venom which he intended to sell (HBP22). Snape jealously guarded the stores in his Potions cupboard, and knew immediately when both gillyweed and boomslang skin were missing (GF27). Clearly if a wizard tried to conjure ingredients, the potion would not come together correctly, or at the best would never reach full strength, so it would be a waste of time. It's possible that ingredients can be increased, just like food, but that is not shown in the canon we have at the present time.
- Body parts can be changed in magical ways, but there are definitely limits to what transfiguration can do. Harry's bones could be regrown overnight using Skelegrow (CS10) and Hermione could shrink her teeth without braces (GF23), but those may have worked because the children were still growing and changing anyway. George Weasley was an adult when his ear was cursed off with Sectumsempra, and it could not be replaced (DH5). Mad-Eye Moody's magical replacement eye, missing nose, and chicken-claw foot are probably clues that Gamp's Law does not allow for full body-rebuilding (GF12). There was almost nothing Madam Pomfrey could do for Bill Weasley's painful scars when he was attacked by Fenrir Greyback (HBP30).
Gamp's Law seems to be a scientific axiom rather than a law passed by the Wizarding government, although this is never specifically stated.
From the Web
The Five Principal Exceptions to Gamp's Law of Elemental Transfiguration, by David Haber, from Beyond Hogwarts