Zakum Diamonds!

Well, the quest is at least partly glitched: the first time you initiate it, it seems to "complete" immediately. Clicking on Adonis gives you 500 exp and he says something like "Remember, you can only get the diamonds if you fight Zakum".

Then you initiate the quest again, the dialogue goes a little different (at a certain point, you can't select an answer. The only one available is "I don't know you, I'm out of here". He continues talking anyway) and the quest remains open.

So, it wouldn't surprise me to know the quest isn't implemented right, and diamonds aren't dropping.

Another possibility is that the diamonds won't drop unless you kill Zakum with one of the iTCG artifacts Adonis mentioned (Miner's Helmet, El Nathian Cloak, etc.) already in your inventory.

Already tried, didn't work. And I fail to see how item drops are dependent on what's in your inventory anyways...