The harder you try to get Diamond the lesser the chances are - this is ofc not scientifically proven, but i think "Murphy's law" is strong in this one! I bet if you more or less give up on it, and aim for something completly different, THEN that Diamond will pop up and grin in your face! Well, maybe you've tried this already, at lvl111....still, i believe in this backwards method, and i'm certain you'll get your Diamond!