Some time has passed since I've played; but most of the blackdiamonds are either inside the scourge dungeon or xerxes dagger dungeon (sorry, I forgets which). I have found a few elsewhere; but those were random.
I've found a black diamond every time in the ancient maze. You don't actually have to go in the maze, but at the entrance to it there are little walled areas to the right and to the left of the main path. The walled area to the right has a magic chest in it, and the walled area to the left has some coins, and three gems -- one of them a black diamond.
Thanks folks. I never had this problem before, but in previous games I'd already gone to the ancient maze and the xexes thing by the time I needed a black diamond. I'll try that today.
In my latest game, I can't seem to find a black diamond. Any ideas where one is likely to turn up? I tried the Wheel clan and Tarant's theive's guild, but several restarts and no dice.
Thanks for all the advice. Most of the locations were random generated, including the Xerxes cave, but the ancient maze has one everytime. Yes, I got it and used it for the God's quest.