Even when the Diamonds are available in the game you have like only 5-10% chance of getting them. You will know when the Diamonds are available, since Rockstar will post it on the load in screens for GTA Online when you load into the lobby.

Diamond Casino Heist Payout: $10.34 -R$11.37 Million\n\n The Diamond Casino Heist payout is by far the highest in GTA Online. The rewards for the mission add up toR$10.34 million on normal difficulty, and it can go up toR$11.37 million, if the heist is on hard difficulty.