In Harry Potter and the Deathly Hallows, Hermione duplicated a locket using the Geminio charm. Why couldn't someone use the Geminio charm to duplicate money? Or do they have seals on them like muggle money and someone would find out?
It's perfectly possible to duplicate gold currency but it
- breaks the law
- has a tendency to evaporate
- can be detected with trivial ease
JKR spoke (tangentially) to the first two issues in an interview in 2000
Q: It seems that the wizards and witches at Hogwarts are able to conjure up many things, such as food for the feasts, chairs and sleeping bags. . .if this is so, why does the wizarding world need money ? What are the limitations on the material objects you can conjure up ? It seems unnecessary that the Weasleys would be in such need of money...
A: Very good question (well done, Jan!!). There is legislation about what you can conjure and what you can't. Something that you conjure out of thin air will not last. This is a rule I set down for myself early on. I love these logical questions!
In "Goblet of Fire" we see the results of trying to duplicate gold. Hagrid has created (or acquired) some 'leprechaun gold' for training purposes...
‘Well, let’s check how yeh’ve done!’ said Hagrid. ‘Count yer coins! An’ there’s no point tryin’ ter steal any, Goyle,’ he added, his beetle-black eyes narrowed. ‘It’s leprechaun gold. Vanishes after a few hours.’
and Ron tries to pass off some of the leprechaun gold he collected at the Quidditch World Cup to pay Harry for his omnioculars, with much the same result...
No,’ said Ron shortly. ‘Why didn’t you tell me about the gold?’ ‘What gold?’ said Harry. ‘The gold I gave you at the Quidditch World Cup,’ said Ron. ‘The leprechaun gold I gave you for my Omnioculars. In the Top Box. Why didn’t you tell me it disappeared?’ Harry had to think for a moment before he realised what Ron was talking about.
‘Must be nice,’ Ron said abruptly, when they had sat down and started serving themselves roast beef and Yorkshire puddings. ‘To have so much money you don’t notice if a pocketful of Galleons goes missing.’ ‘Listen, I had other stuff on my mind that night!’ said Harry impatiently. ‘We all did, remember?’ ‘I didn’t know leprechaun gold vanishes,’ Ron muttered. ‘I thought I was paying you back. You shouldn’t’ve given me that Chudley Cannon hat for Christmas.’
In 'Deathly Hallows' we learn that goblins have the ability to instantly detect fake or adulterated currency;
The long counter was manned by goblins sitting on high stools, serving the first customers of the day. Hermione, Ron and Travers headed towards an old goblin who was examining a thick gold coin through an eyeglass. Hermione allowed Travers to step ahead of her on the pretext of explaining features of the hall to Ron.
The goblin tossed the coin he was holding aside, said to nobody in particular, ‘Leprechaun,’ and then greeted Travers
Leprechaun gold aside, there is an example of the creation of imitation gold galleons, which did not fade away for a long time (over two years, at least). They are the
fake galleons Hermione created to notify members of Dumbledore's Army of meeting dates in Order of the Phoenix.
They were mentioned as still being in use in Deathly Hallows, and did not seem to have deteriorated in quality at all.
They seemed to bear a reasonable resemblance to ordinary Galleons. Harry, at least, noticed no obvious differences, and thought it might be possible to accidentally spend one.
They were almost definitely illegal, though it's never mentioned.
So, yes, it is possible to create fake money, and, though the Geminio charm is never mentioned, it could possibly have been used. Otherwise perhaps they were buttons transfigured in to Galleons.
Unlike what Richard said, however, they don't always evaporate, and are perhaps not so easy to detect.
Yes its possible, though it would probably be useless to try and do so, as Goblin's are able to tell the difference between Wizard-Made Item's and true Goblin-Forged Items (As Griphook was able to authenticate Gryffindor's Sword.) And since Goblin's control the Banking System, that kind of makes it worthless to try because they probably have specific things they mint the coins with that makes it extremely easy for them to spot duplicates making it worthless to even try. Plus, Goblin cast or forged items likely have unique and hidden properties that couldn't be duplicated using Wizard magic.
However, if a Wizard/Witch was clever enough, they could duplicate Muggle Currency (which most Wizards likely wouldn't have a clue about and likely wouldn't bother with) which in turn could be "cleaned" if the person is willing to do a bit of illegal money laundering to switch out the duplicated currency with non-duplicated currency which could then be exchanged at Gringotts without anyone being the wiser. Though I imagine most wouldn't even think to try it except maybe an unscrupulous muggleborn or half-blood with the know how to pull off something of this nature.
It is also worth noting that things copied by the Gemino charm are worthless as mentioned in the scene of Deathly Hallows in which the trio break into Gringotts:
"They have added Gemino and Flagrante curses!" said Griphook. "Everything you touch will burn and multiply but the copies are worthless."