Just wait for happy hours (which last all day) and your gem cost is halved or even with the special happy hours just one third of normal cost.

And for the endless coppermine, it's the ultimate luxourious (sorry Dutchy here, i know this is probably spelled wrong) building to add to your Island.
(King-Fero you don't háve to throw baskets on there :PPP ) I was contemplating between building last available gempit or reducing my copper ore bottle neck and have chosen the latter.
Since my economy doens't run smooth (i will never be able to buy 40k copper ore in one go) and buying frequently and depleting my buying options ( i don't have stacks of goldcoin) didn't feel good, the endless coppermine was an option for me.