Wow! I just got my 5th Diamond dragon! I used Brightsteel + Bat in an effort to get various metal types.
To those still trying to breed their first, don't give up! I didn't get my first Diamond until I was almost lvl 100 and had already been playing for almost a year. Now I have 5 diamonds and 7 hybrids all through breeding.
Here are my winning combos so far. I hope this helps someone!:
1st- Deep + Genie
2nd- Quetzel + Headless
3rd- Bat + Amethyst
4th- Hermes + Goblin
5th- Brightsteel + Bat