I hope at some point we'll know why his price for emeralds is so much higher than usual. There should be a reason. He either crafts some legendary emerald gear, or maybe imprisons innocent child souls inside those shiny gems first (and maybe use em to craft some legendary gear then), or does other creepy things. All wizards are bastards.
You know the Minotaur Oracle in the labyrinth who's entrance is on the same map will give you a vision (5k exp) in exchange to 10 large emeralds.
I'd say he's stock piling the small ones in case the crafting patch allows you to merge small gems into large one and using the large ones to level up with less risk as he's actually playing on Ironman... or its just a game perk.