A large number of questions and discussions on Wizarding economy seem to take it for granted that Galleons and Sickles are made of pure Gold and Silver respectively, but is there actually any statement from JKR to this effect?
I'm looking specifically for any statements made by JKR herself in the books, in interviews or via Pottermore, regarding the purity of metals in Galleons and Sickles.
I’m fairly sure there are no canon statements about this.
I’m not aware of anything in the books, Pottermore or an interview statements which explicitly describes wizarding coins as being made of pure gold or silver.
There are statements on their size, mass and value relative to Muggle currencies, but I have nothing about their composition.
I would be happily corrected on that.
Speculation:
Personally, I doubt you could have currency made of pure silver or gold. They’re both very malleable (soft) metals, and I think any coin made only of those metals would be too soft to be practical. Of course, that doesn’t stop the magical folk applying charms to the coins to stop them falling apart. But I am not a chemist or a coin expert.
ETA: And Martha points out in the comments that sometimes the malleability of gold made it a very desirable substance for coins. Like I said, I am not a coin expert, so I don’t know when pure gold coins dropped out of general usage. A description of a Quidditch Match in 1269 tells us that coins called Galleons were in use in the mid-thirteenth century:
According to Madam Rabnott, Bragge brought a caged Snidget to the match and told the assembled players that he would award one hundred and fifty Galleons (equivalent to over a million Galleons today) to the player who caught it during the course of the game.
– Quidditch Through the Ages, Chapter 4 (The Arrival of the Golden Snitch)
As with some Muggle coins, this is probably a different physical form to the modern Galleon, with merely the name carried down. Perhaps (as per Martha’s comment) early Galleons were pure gold, but I still suspect this might not be the case with the modern coin. /edit
The HP Lexicon has a good essay on wizard money that attempts to pin down the size of Galleons and Sickles, which assumes different levels of purity. That suggests that gold or silver coins are possible, but obviously HPL isn’t canon and that doesn’t prove that they are.
Canon says nothing on whether they are true gold and silver. In The books Things are typically exactly what they say they are, and since according to the book wizards separated from the muggle world in the 1600's we see that they still use things exactly like they were in medieval Europe which leads to my Assumption below.
--Assumption--
They most likely are pure gold and silver, As that is how gold and silver coins where used for thousands of years across the world. The worth of the coin is in the actually weight and purity of the metal. In other fantasy and movies you see people bite gold coins to see if they are real for the fact that yes you can actually bite them. But since we are in the Wizarding world its not unrealistic to assume the coins are enchanted to make them solid.
--Medieval English coin--
"The Florin or Double Leopard was an attempt in 1344 by English king Edward III to produce a gold coinage suitable for use in Europe as well as in England (see also Half Florin or Leopard and Quarter Florin or Helm). It was 108 grains (6.99829 grams) of nominal pure ('fine') gold" --- Cited from Wikipedia
