If you look up "Jewels" in the Inventory Guide of The Hub, it will show all the places/items that drop them. And what level are your Jewelers? I believe the higher the level, the more likely to drop. None of mine are a Level 9, so I can't say if the Level 9 building always drops them or not. But i believe my Level 1 often doesn't and the higher ones do (I have 3). I know my Level 9 Sawmill drops more/better stuff than a Level 7; same for my Mines.