While purchases are normally visible instantly on your account, sometimes payments might be delayed for as long as 24 hours. If you have made the purchase longer than 24 hours ago but are still missing your gems:

For iOS: please contact iTunes support here
For Android: please send us a screenshot of your receipt, with the transaction ID clearly visible (it looks similar to GPA.1234-1234-1234-12345)
Sorry for the inconvenience, please let us know if you need further assistance.