Hello Ms. Q,
"why do we use coins?"
I know,----why do we use coins? There are many reasons for that, one is to "purchase" something spiritual ie graveyard dirt, or to barter with, or to ground or anchor with, such as using it with railroad spikes.
But the use of coins is much, much older.
Here in the States we use either Mercury Dimes or Indian Head pennies, either silver or copper as these are underground metals and are used in the construction of magical materials.
If you are going to use silver or copper coins it is best to buy what coin dealers call "sliders" these are coins so worn down that its difficult to see or read their markings and as a collectible coin almost worthless except for the metal (silver/copper) content. So I would buy "sliders".
Now if this is a foreign country it still has to be a coin with silver/copper content but the image has to be different, such as a solider or famous military person or a powerful symbol, such as the Mexican peso with the image of the eagle and the snake, one has to apply a symbol for the type of power one wants to generate for whatever type of use.
There is a lot more to this but I'll have to again "dig" through my books and notes of what my Granny said about coins.
I'll post on that in greater depth later.
Blessings Ms. Q