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
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