I went and purchased diamonds from the website 5 hours ago and I'm still waiting for them to show up. Its cleared my bank and everything so I don't know what the heck is the hold up. When I paid, it said it could take "up to 1 hour" for them to show up and that I may have to log out for them to show up. I've waited well past an hour (like I said, its been 5 hours now) and I've logged out of the game completely every 20 minutes and still nothing.
First time payment via Paypal?
If so, it can take up to 24 hours for the first time only. I believe each transaction's manually checked by Skrill/Moneybookers, who handle the payments for Gameforge.
If you did not pay via PayPal, you may want to file a support ticket and include the payment numbers - it's possible that you accidentally sent the diamonds to the wrong server or account, and they can find where the diamonds went and correct this for you.