I read Tolkien’s canon (The Silmarillion, The Hobbit, The Lord of the Rings) every year about christmas. This year’s pondering is over Balin and Gandalf and Thrór’s ring.
Thrór possessed one of the seven rings that the dwarves got from Sauron of old. Inherited from father to son through generations, it was an heirloom of immense value for the Durin line. It passed to Thrain, who was Thrór’s son, and Thorin Oakenshield’s father. When Sauron woke again during the Third Age, Thrain was taken captive in Dol Guldur, and the ring taken from him. He perished there before Gandalf could resuce him. All this Gandalf told in the council of Elrond.
Now, by the same council, Glóin reveals that one of Balin’s main reasons for attempting to recolonize Moria, was to find Thrór’s ring. But Gandalf knew that it was not in Moria, as it was taken from Thrain in Dol Guldur. When Gandalf knew this, it is quite obvious that Thorin knew too. Gandalf would not keep information hidden about Thrain’s condition and death from his only son. So both Gandalf and Thorin must have known that Thrór’s ring was taken. Still, Balin, did not know, even though he was a close friend and companion of both Thorin and Gandalf. Consider the last scene in the Hobbit, where Gandalf and Balin, on a journey all the way from The Lonely Mountain, visit Bilbo. It is a meeting between close friends. Yet, Balin knew not. So he went with his followers to seek for the ring, and the whole colony was killed cruelly, fighting a last stand against the orcs of Moria.
In retrospect, a bit more openess about the ring would perhaps have been advisable. But the keeping and the keeper of the ring was constantly kept a tight secret in the Durin line. No one knew for sure who had the ring, until it was given to its next keeper. The appendices tell us that the dwarven rings were treacherous. Though not making the dwarves into shadows and slaves of Sauron, the ring keepers of the dwarves became jealous, and a constant hunger for more gold was set in them. Thus, the ring was often the base for a large hoard of treasure, which in turn could cause grieves like wars and dragon plunder.
Perhaps Gandalf considered this, when he kept his knowledge about Thrór’s ring hidden. It is still a bit of a mystery to me though.