<name>, you've already proven more able than all my apprentices combined.
The next parts I need require a little more strength. Large stone slabs can be found on rock elementals if you're careful enough. Their bodies are stronger and larger, and therefore more prone to having pieces large enough for my tests. You can find them further west of here or southeast along the mountains' edges.
Take your time; I'll need a while to test these smaller pieces you brought me.
I'm most pleased, <name>. My spells affected the smaller pieces just as I predicted. If this experiment is as successful as I foresee, then the elementals I'll be able to control will put anything summoned prior to this to shame.