Buying gems is usually seamless and quick. Sometimes the response from Apple iTunes server may get delayed. When this happens, just wait a few minutes. Your purchase is not lost, and once the game connects to Apple's servers it will come through.

If the transaction still doesn't complete, verify that your device is connected to the internet and restart the game. Double tap on the Home button to show a list of running applications. Swipe up the game to close it, and open it again.