A Stone is a Goblin's Best Friend
Head to this marker for another side-quest
You will find Kachoo, a goblin who is down because he lost his previous stone. Offer to help him and he gives you some idea of where he think it is.
Head to this location, the opposite hallway to Kachoo.
The bookshelf that has a glowing blue light on it is the one you should be looking for.
Inside you will find a note and his shiny stone.
Return to Kachoo and give him the stone to submit the quest.