Playing the mini-game allows you to unlock a lot of items, including diamonds.

The available mini-games are:

1. Fan Gifts 
2. Fashion Arena 
3. Lucky Slots 
4. Golden Spin 

They are unlocked as your level increases. The first mini-game, "Fan Gifts," unlocks at level 9. The rest opens at 10, 12, and 23, respectively.

You can locate them by clicking the Mini Games icon at the top right corner of the game home screen.