It might simply be that iTune’s servers are overloaded, depending on their workload and the time of day. In consequence, in-game purchases can be greyed out until the servers are back to normal. This issue is usually fixed after a short period of time.
In case you are experiencing problems with purchasing in-game currency over an extended period of time, please give the following suggestions a try:
- Exit your game and restart it.
- Log out and back in with your iTunes account. In order to do so, go to your device settings -> iTunes & Appstore
- Enable in-app purchases on your device under Settings --> General --> Restrictions.
- Make sure that the time/date settings on your device are correct.
- Make sure that the information about your payment method for your Apple ID are correct and that your credit card’s verification code (if any) has been entered.
- Use the same Apple ID to purchase diamonds as the one you used to download the game.
If these don't help, we kindly advise you to to contact iTunes Support, since all purchase-related issues run through Apple' support first.