I'm working on breeding a Diamond so far this is how it's gone.
First I looked for a combo with a limited number of possibilities to maximize my chances at each stage. The combo that seemed best which included several dragons I didn't have along with Diamond and low breeding times except for Diamond was Metal+Goodwitch(Only 10 possibles with a max of 17 hours and commons with only 5 & 12 hours).
I had a Goodwitch, but not a Metal dragon.
I bought a Light dragon for 360,000 I believe it was to start. I already had a Winter dragon. I chose those to breed the Metal as it only had 5 possibles and I only had the Winter prior to that of those possible(Note: Light + Water works as well, but my Water is in storage). I took a few tries, but I got a Metal dragon out of it and make sure my Evolution Temple was free for when it hatched.
Once I got my Metal dragon so far the Metal+Goodwitch results have been:
Winter, Cloud(a new one for me), Love (Note: I still need 4 out of the 10 possibles, so here's hoping..)
P.S. I believe I got my Goodwitch through Winter+Fire(It also can breed a few other nice combos as well).