Stupid Quarry!
They certainly have an idea of how many tons of gravel fit into a standard 10 yard truck! Sounds like they don't want to be bothered.
Anyway, there are about 100 lbs in a cubic foot of gravel. That makes about 2700 lbs in a cubic yard. So, in your case, you want about 8000 lbs or 4 tons.
Good Luck!
Dodgeman