Many kinds of magic withered and died in the 20th Century; Ghost Coins, in contrast, blossomed in power. It’s one of those ‘power in contradiction’ things; it used to be that even when a nation fell apart its coinage would still have some inherent value. After all, a Byzantine gold solidus still had precisely the same amount of gold in it the day after Constantinople finally fell as it did the day before. But with the rise of paper money and coins made out of non-precious metal, well, once the country was gone then so was the value.
Or, rather, the exoteric value. It turns out that there’s quite a bit of esoteric power to be gleaned from individual units of currency; after all, money itself is a symbol given tangible form. It gets tricky when the currency itself has value as an artifact — for example, authentic Confederate currency is a legitimate collector’s item, and thus cannot keep this kind of occult charge — but there are any number of failed currencies out there that can be used as Ghost Coins. The demonetization of European national currencies as part of the conversion to the euro alone has created a stable ‘currency’ pool from which to work.
So what do Ghost Coins do? Well, people use them to ‘purchase’ and ‘sell’ things that cannot normally be bought or sold. One problem that has always bedeviled magicians is how to guarantee magical contracts. There are no Wizard Courts, nothing like a magical law enforcement system, and certainly there will never be anything like occult prisons: mages collectively stamp down, hard, on anybody who tries to set anything like that up. Mages may like each other, may work with each other, and even learn to rely on a specific fellow-mage — but they never, ever trust each other.
Thus the use of a Ghost Coin. Each mage involved in the transaction touches a Coin and imposes a if-then statement on it (for example: “If Bob casts The Five Nights of Surcease upon me I will give unto him the knowledge of the Sunrise Water Ritual” / “If Jane offers the Sunrise Water Ritual to me I will release The Five Nights of Surcease upon him”). Once all the if-conditions are met, the coin triggers the then-effects — and is consumed by the ritual. Simple, straightforward, and resistant to chicanery.
Not entirely immune to chicanery, mind you. There are ways to game the system. But for most occult transactions Ghost Coins work perfectly well. And the mages who do play games with the Ghost Coin ritual will soon discover that they have a fairly nasty reputation, and nobody will want to work with them without getting paid in advance. Which is sort of ironic, no?