From one of the WoB:
Q: [Not sure on this] Is there something important about the stone, and the girl that's missing that has to do kind of with the stone, and how Tien gives him rocks?
A: Some of that you're reading too much into, but something is real. I'm not going to tell you what is what. But there's definitely something about that
Awkwardly phrased question aside, Brandon says that there is something to this question. The part I think he was talking about was Tien with the rocks, which stood out to me to begin with. I think there definitely was something special or different about tien having to do with rocks or stone, but don't think it was down syndrome or anything like that.