This is new multi-stage feature that was originally released on September 11th, 2012. If you're having trouble earning enough Gems to finish the Gem Tower feature, you can now complete the limited edition Gem Mine feature that has shown up on the outskirts of your farm. This Gem Mine comes with multiple stages to complete, and you'll receive gem-themed prizes for completing each stage. You'll also apparently win gems for the Gem Tower itself, according to Zynga.