If you are new. Spend it on sumons (2700 diamonds 10x sumons), and change your wishlist. For end game players (chapter 32+), you can do fast reward free, 50 diamonds, 80, 100, 100, per day. Buy everything for gold in store, except si materials (just if you need), refresh the store 2x per day (100+100 = 200 diamonds), and do sumons/choice sumons, or save diamonds for stargaze if you don't need 4f heros.
