Diamonds can be earned by accessing tapping on the '+' next to your Diamond count on the top of your screen. Then scroll to the right until you see a button that says "Offers: Earn Free Diamonds". This will take you to TapJoy's offer wall.
Please remember, any offers completed are done so through TapJoy. If you do not receive your Diamonds after completing the offer, please make sure to contact TapJoy through the offer wall within DragonSoul (you can do so by taking the same steps, shown above, to get back to the TapJoy offer wall in DragonSoul, then tapping on "Missing Diamonds?" at the bottom right of your screen). Contacting PerBlue will result in us directing you to TapJoy so save yourself a step and go straight to the source!