I stole Vacca's green gem from his cave on Sentinel Island, and when he said give it back I said no and killed him. So I failed the quest but I figured, "Hey I got this gem that Laiken dropped, should be good for something....". But so far I haven't found a single use for it, and it is in the regular inventory not the quest items. It's worth 5000 gold so I'm thinking about just selling it unless there actually is a good use for it...... Anyone know???