Most gem purchase transactions go smoothly. However, it might be possible that single deliveries or requests from the App Store get delayed or result in errors. This is why it can occasionally take a little while for your purchase to be delivered, so please be patient just a little longer and don't lose hope.
In the meanwhile, you should never try to carry out the same purchase again. As your previous order might still be processed, you will mostly likely end up getting charged multiple times. Please note that it is not possible for us to refund such additional purchases.
What you can do instead is to try restarting the game and see if the server just needed a fresh connection in order to deliver your purchased gems.