How to breed an Emerald Dragon

Generally they will breed a child of the same exact type, at least 95% of the time in my experience. However they can breed a hybrid of their elements.

When breeding two Chrome dragons (air/metal) repeatedly I found that once I got a Rust dragon (Metal+water)! This is because, apparently, on rare occasion an Air dragon breeds as a fire or water dragon instead of an air dragon.

Yes, the breeding time is always the same as hatching time for all dragons, rare dragons included. This means you can check your breeding time to know what dragon you're going to get; you're getting a Tree Dragon in this case.

You can verify the hatch/breeding time for any dragon with the Wiki's Dragon Reference Chart to check which kind of dragon you might be expecting. Alternately you can just look in the store in-game. Look for dragons matching some/all of the elements of the Parents (combined) that have the same hatch time as your remaining Breeding time.

Note that the upgraded breeding cave/island reduce your breeding time by 20%, but the incubation time will remain the same.