After talking to the dwarven jeweller, I didn't get an actual brooch showing up in the inventory, either, but could deliver it to Sir Dante's servant to complete the quest, and return to the dwarven jeweller for a reward (1500 gold).
There is a 'Brooch' in his trade inventory with no special stats (just durability and weight), which matches the one I get loading my save before entering the Dwarven Halls (in that case, it doesn't show up in the trade window). If leaving the Dark Forest and returning hadn't reset something enough to complete the quest, I would have suggested buying the brooch and see if that would help.
