The Genies can award up to 200 diamonds
The percentage chance of getting any diamonds is small (I think much smaller than for wishing wells): 50 diamonds 5%, 100 diamonds 2%, 200 diamonds 1%. For someone like me who reads those as 'there's a 95%, 98%, and 99% chance you're
not getting this prize', they look especially small, lol! Nevertheless, I have received all of those denominations at some point in the past 1.5yrs. The most frequent I think is the 50 (which would make sense). I just keep one Genie out, may look into putting out a couple in the guest race space when I'm next 'between chapters' (since that's usually a few months for me, it should work). I have some old Wishing Wells still in inventory, too that could go out then, too.