Board Thread:Lore Discussion/@comment-7153552-20130708162914/@comment-3573184-20130910061038

Given that Snow-Throat was, at least according to The Book of The Dragonborn, rendered kingless and bleeding, implying it was deactivated, and that this had to happen before Alduin returned, the stone being anything we see in-game is unlikely.

Alduin and the dragons only returned after it was deactivated. The Eye of Magnus and The Skyforge have, to our knowledge, not changed in recent times. The Thu'um hadn't been largely used in ages. Making all of them being the stone unlikely.

The stone most likely would have been something related to the High King, perhaps one of the jewels on his crown, which could have been cracked or destroyed when Ulfric shouted him down during the battle.