Unlocks at level 10. You earn diamonds when you improve your highest ranking attained. You can check it @ Arena -> Rules. Diamonds given increase exponentially as your climb higher rank. Meaning to say, the diamond u get from Rank 200 -> Rank 100 is way more then Rank 1000 -> Rank 900.
Important: Save your dragonscale and arena point for heroes. Also you can refresh the shop for 10 arena chips (so if you have 1010 chips you can buy a hero twice!)
Rationale: These heroes are the easiest to get 5* as you can consistently get 5 ss from each exchange. If you are free to play, it is highly likely that these heroes will be your first 5*. Don't waste them on equipment as the benefits of getting 5* hero is better.
Golden tip: The arena reward are given out daily at 9pm (GM+8), leave your last arena chance and refresh to a winnable opponent, 30 seconds before 9pm, attack and hope to win! If you win you secure a higher ranking for more rewards!