Doing this would have a severely negative effect on the value of gold coins. Silver would actually, in the long run, become more valuable than the gold.
It's just like the reason behind why coins used for rewards (by TK, Mur, Chew, everyone) are recycled and not just created out of thin air.
Diamonds have no actually determined value at the moment, gold coins, on the other hand, do. So it's much much harder to do something like this without ruining the entire market.