You were banned by Opticalza (The *owner* of the server) for 14 days for "Stealing huge amounts of diamond from SirDustin". It is quite unlikely that Opti will lower the length of your ban, as he has zero tolerance for griefers, but wait for his reply.
Whether SirDustin used "mods" or not isn't your concern, it's the ops/admins/owners duty to look into these things. Also, have you ever heard of the "Fortune" enchant on picks? You can get 4 diamonds per one ore mined... I've run into six-ore veins of diamond... that's almost three full blocks of diamond in one vein. His diamond is legitimate. Your attempt to steal it is not.
As far as "Tell me you wouldn't grief that!!!" goes, I hope beyond all hope that you don't have this attitude toward everything in life... What you just said about a game translates roughly to "Tell me you wouldn't take advantage of her!!! I mean, she was wearing a short skirt, high heels, and that top..." in the real world.
Your logic is flawed, I sincerely suggest you take a step back and examine your outlook on life and the consequences of your actions and words.