Use your blue gems to top the energy meter right up or wait till it refills by it self.
Tap on the energy bar, then select Invite. From here, you can simply send an SMS text to your own mobile number as many times as you want. The game doesn't check that the recipient of the text did anything with it, and you'll be instantly rewarded!
Unfortunatelly there's only one way to get hold of more blue gems and that's to hit up the in-app purchase store.
Smash any pots you come across as these very often contain extra silver coins.
In the bottom right-hand corner of the screen there's a little tab marked 'Get free coins'. Click on this and you'll be taken to a list of promotional events and advertising you can participate in to get rewarded with a pretty decent chunk of change.