I have done these heists many times now and have yet to get diamonds.
That doesn’t mean it isn’t possible.
When you look at the awards for the heists there is one that is given out for all types of “loot”. Since I have gotten Cash Gold and art I still haven’t gotten the award.
Logically there must be a fourth option and it’s safe to assume that the fourth option is Diamonds.
There must be something that keeps it hidden until you have done the heists a specific amount of times.
I have yet to delve into the game code as of yet but I’m sure that after awhile Diamonds will show up as an option.