Gemstone x Lotus = Zombie
I did try this last combo and got only coral, no zombie
Gemstone & Lotus gives 20 possibilities! If all you want is the Zombie i would strongly recommend a more targeted breed. There should only be three possibilities: Poison (and probably lots of them, based on my own track record), Familiar, and Zombie.
Your chances of getting Zombie will go way up if you go simpler. Breeding like this, i've gotten Witch, Boo, Zombie, & Mistmoth.
Good luck!
I do agree there are better ways to get zombie. I was attempting to get Onyx. Just wanted to post another working combo that may give a welcome fail.