Dig Deep (UP)

Each island has 4 diggers, three you buy for gold coins and one you buy for diamonds. The last island price for the diamond digger is 20k. Diamonds are relatively slow to acquire so this was the very last achievement for me.

As Woggo said, gems comes very slowly in this game. After building all 12 statues, money won't be necessary to progress. So, every in-game day you are prompted if you want more coins or gems. Always choose gems.
In order to make this achievement a little less tiring, the best pets to earn more gems are Hamster, Chipmunk and Skunk. Choose them. Then, hire your last digger when you get 20480 gems.