I just got it
After many fails (including lots of nights, a moon, wizards galore and corals) i finally got black diamond!
Combo was dark angel/night elf, 24 hours
Pic of egg
Good luck everyone
Edit
Ok, I was super excited to finally have black diamond, but my excitement rapidly went down the gurgler when I discovered I can't actually house it TL, please help! We shouldn't have to beg like this