How does Gandalf know that dragon-fire might be able to melt the rings of power, but not the One Ring?

Question

In Fellowship of the Ring "The Shadow of the Past", Frodo asks about destroying the One Ring. Gandalf says:

It has been said that dragon-fire could melt and consume the rings of power but there is not now any dragon left on earth in which the old fire is hot enough, nor was there ever any dragon, not even Ancalagon the Black, who could have harmed the One Ring, the Ruling Ring, for that was made by Sauron himself.

How does Gandalf know this? Did any of the 19 rings of power get destroyed by dragon fire in the past? If not, then what makes him think that the rings of power can be destroyed by dragon-fire, but not the One Ring?

Answer 1

The issue of what Gandalf knew concerning the destruction of the One Ring is discussed in the answers to How does Gandalf (or anyone) know how to destroy the Ring? As for the lesser rings, Gandalf states not only that these could be destroyed by dragons, but that four were in fact destroyed in this way.

Seven the Dwarf kings possessed, but three he [Sauron] has recovered, and the others the dragons have consumed.

("The Shadow of the Past")

This is supported by the following quote from "Of the Rings of Power and the Third Age" (The Silmarillion):

It is said that the foundation of each of the Seven Hoards of the Dwarf-kings of old was a golden ring; but all these hoards long ago were plundered and the Dragons devoured them, and of the Seven Rings, some were consumed in fire and some Sauron recovered.

None of this brings us any closer to answering the question: how did Gandalf know this? Unfortunately, we know next to nothing about the seven, except the ring of Durin III which was seized by Sauron when he captured Thrain II around TA2845. We do know that dragons reappeared on the Withered Heath around TA2570, and that the ring of Durin III was the last to be lost (see the entries in "The Tale of Years"). That gives us roughly a 300 year window in which four of the seven were destroyed by dragons. Gandalf arrived in Middle-earth around TA1000, so he was present the whole time, and had plenty of time to investigate (the Council of Elrond took place in TA3018). It could be that he simply found eye witnesses. If the dwarf kings kept the rings on their person, then there are probably only two outcomes when a (fire-breathing) dragon invades:

• king gets incinerated and ring is destroyed

• king escapes taking the ring with him (only for it to fall into Sauron's hands at a later date)

Of course, if the ring was hidden away deep in the vaults when the king was incinerated then it's hard to see how Gandalf could have been so sure of its fate. There is another, less prosaic possibility. In "Of the Rings of Power and the Third Age" (The Silmarillion), we find the following:

But the elves were not so lightly to be caught. As soon as Sauron set the One Ring on his finger they were aware of him...

(see also "The Council of Elrond" and "The Tale of Years" entry for (c) SA1600). Thus the bearers of the Three could sense the power of the One, and may similarly have felt some connection to the Seven, at least enough to sense their destruction.

Answer 2

Gandalf didn’t say that dragon fire could destroy rings of power, he said that it had been said that dragon fire could destroy them. He was just passing on hearsay. How he knows it is presumably that at some point in the previous three thousand years he heard someone say it.

Answer 3

Because all of the first dragons were bred by Morgoth during the First Age, including Ancalagon the Black, the most fierce of all dragons. Since Sauron inherited the dominion over Morgoth's realm, it's not unreasonable to assume the One Ring was created to be impervious to the servants of Sauron, since he poured part of his being into the One. He apparently did not make the other Rings of Power impervious to dragon fire, as four of the Seven dwarf rings were destroyed in by dragons attracted to dwarven treasure hoards (in the same way Smaug was attracted to the Lonely Mountain treasure) during the Third Age.

Note that the Elvish Rings were crafted by Celebrimbor and not by Sauron, so they might or might not be considered impervious to dragon fire as well.