Return the Dragon Stone to Farengar Secret-Fire. He will only accept the Dragonstone after the Dragon Rising quest has begun. If the stone has been obtained before Bleak Falls Barrow has started, then it can be turned over to him immediately.
Presenting the Dragonstone to Farengar automatically starts the Dragon Rising mission. Upon conclusion of Dragon Rising, the game world irrevocably changes and the Dragonborn is thereafter subject to random dragon attacks, which not only makes travelling around Skyrim hazardous (especially for low-level players), it often results in the death of non-essential NPCs who may still have quests to be completed, thereby denying the Dragonborn of gold and/or other treasure. It is therefore not recommended for low-level players to return the Dragonstone until much later in the game when as many side-quests and radiant missions as possible are completed and Skyrim thoroughly explored.
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