They're a limited-time reward, meaning Rockstar will routinely add them and remove them as they please.
The big issue with Diamonds is that they're not always available as a reward from the vault underneath the casino. Below you can see how much more Diamonds payout compared to other everything else:
These shiny gems pay out wildly more money compared to the likes of Cash, Artwork or Gold. You see one of the game's most profitable commodities to steal during heists (outside of the Cayo Perico Panther Statue) is Diamonds. How To Get Diamonds in GTA Online Casino Heist? With regards to the Diamond Casino & Resort Heist, there's also one other thing to factor in. How to get guaranteed Diamonds in GTA Online Casino Heist
