Hmmm, tricky question. One suggestion would be to check out
www.volberding.com for a spoiler/walkthrough.
Anyway, I think I know where the missing dragon shard might be located. Close to the room where you find Hephaestus, the blind gnome, there is a lift going up. If you take that one you'll find a hole in the floor. Drop through it. In the area where you are now you'll find the divine sigil (which I belive that you have already), in another dicrection you'll find a broken bridge over a pit full of lava. You'll have to jump between the pillars of that broken bridge over to the other side. There you'll find a maze with some chest (some of them are difficult to spot due to the darkness), and the dragon-shard should be in one of those chests.
Another possibility is that you've found the dragonshard, but the inventory has been full and you've dropped it on the floor. In that case you'd have to search the vicinity of all the chests.
I hope this will solve your problem.
Good Luck // Jazzid