Diamonds appear in the Cornucopia fountain every 20 hours.
When your fountain is charged, you can click on it to see the details:
The timer in the middle shows how long until diamonds will appear, or the time you have to claim them if they've already appeared.
Note
Diamonds will be lost if you don't collect them within 24 hours after they appear in the fountain!
In the top left you can see how many charges you have, or how many times you can collect diamonds before they run out.
In the top right, your progress towards the next collection streak gift is displayed.
Reminder
You get a gift containing diamonds and energy when your collection streak reaches 5, 12, and 30.
The timer below your streak counter shows how long your collection streak will last. If you don't claim any diamonds within that time, your streak will break.
Click the button next to your streak counter to see your collection history.