I agree with Delta. This idea would basically scrap much of what PMC is. Diamonds are supposed to be shows of appreciation. And besides, a lot of people don't diamond every single thing they like anyway. Maybe, going off of your idea, there could be a system of maybe limited diamonds based on level, and level 1 accounts could only diamond 50 entries...
But 1 diamond is too few.
Plus, this means that if you do fake accounts, you could over-diamond your own submissions much more, especially if you have friends to help.