- One in the Warrior's Respite (probably the one you found) - One in The Arisen's Refuge. - One in the Tower of Treasons Repaid: First floor You go through the broken bars and emerge out by the red river follow that to the ledge and look your left. You should see a dot on your map that's how I ended up finding it. Jump down and the moonbeam gem should be next to a skeleton.
- One in The Fallen City, head north to the cliff and you'll see a corpse looking off. It's in the corpses lap, you should see the gem while approaching him.
Information taken from here: http://www.gamefaqs.com/boards/626514-dragons-dogma/66037109
I did notice that once a door is opened by the gem it stays opened even on subsequent run-throughs. I also noticed that a chest seems to yield the same item every time (I opened a chest but didn't take the item, and the next time I revisited the area and opened the chest again it was the same item). I wonder if the chest items are fixed or if they are random.
On a side note, does anyone know where you can find Crimson Stones? I've been through BBI a number of times already and no Stones yet.