I've had the same problem as marcb80.  When I go to the enchanter's hut, he tells me that if I bring him 3 Midnight Stones, he can make me an item.  The higher-level version of this quest (the blacksmith who apparently makes a shield) also does the same thing.  I have 5 in my inventory, and I've tried various things, such as having only the unit with the Stones visit the house, but nothing seems to complete the quest.

You may have to turn in the stones at a different location.

This quest doesn't work for me either, the blacksmith tells me to bring him "a couple" stones for a shield, I have 4 in my inventory but the only dialog that pops up is the "bring me a couple stones and I'll make you a shield".

Hmmm it worked fine for me. I wonder why it doesnt work for some but it does for others.

I'm stuck on this quest as well. I had 1 stone before, got quest, went to blacksmith, he says: bring me a couple of midnight stones, I'll craft you a shield". Found second stone, went back nothing. Is there a way to cancel a quest, since it seem s I can't pick up anymore quests and this one is too frustrating.

The stones are all over, even without the quest. Seems like half of the notable locations just give a midnight stone. I just finished the quest on turn 25 or so, with 2 leftover stones.

I had this quest and I ended up getting it to work.  It tied the quest to a specific Champion.  I brought all my guys to a square next to the trigger and put the stones on each guy until it worked.  If that isn't your situation then I'm not sure what you would do.  Maybe the game linked the quest to a regular troop or something.  

Hope that helps.

worked for me.  my sov had all the stones.  every old battlefireld i came across game me one, so I had about 7 by the time i took the quest at the inn.  I just walked over to the blacksmith and he took 3-4 stones and game me some sword I never did use.