I tried this hack on my jailbroken iphone but it doesn't seem to work with latest version of this Game. I even tried downgrading the game to version 2.5.5 but you cant play that as it prompts you to update it the moment you open it.
How did i make it work - (you device need to be jailbroken which assume is ok considering this hack in the first place is for jailbroken devices)
Step 1 - I installed GameGem in cydia,
Step 2 - Follow Gamegem tutorial to change the coins and gem in the game. Bear in mind its very tricky if you have gems below 10 (single digit) as results come out to be very high. And also you need to change the values of coins/gems in between hack. For clarification what i exactly mean, you can watch the following video to see exactly what i mean.
https://www.youtube.com/watch?v=RvwalDm1d1k
I hope this helps. I was able to hack gems for my kid who cant upgrade his dragon on his ipad (non jailbroken). The reason this was achievable was because the game progress get synced with you game center account. Once i have changed coins/gems on my jailbroken iphone i just launched a round and finish it and this should sync your changed number of coins/gems to all your devices.