I think the Ring destroyed itself, and in that Tolkien was saying something about the nature of evil.
The Ring was malicious, and it didn’t want any master other than Sauron. It seems to have enjoyed betraying its bearer. It has rules to follow; it must obey its bearer, but it seems to have some degree of discretion as to exactly how to do that. Sort of like the Monkey’s Paw, the Ring seems to serve its bearer in the worst possible way. That’s how it gets its shits and giggles. Of course, its long term goal is to get back to Sauron, but any petty malice it can institute in the meantime, it does.
On the slopes of Orodruin, Gollum attacks Frodo. Frodo fights him off and
[QUOTE=JRRT in RotK]
‘Down, down!’ he gasped, clutiching his hand to his breast so that beneath the cover of his leather shirt he clasped the Ring. ‘Down, you creeping thing, and out of my path! Your time is at an end. You cannot betray or slay me now.’
Then suddenly, as before under the eaves of the Emyn Muil, Sam saw these two rivals with other vision. A crouching shape, scarcely more th shadow of a living thing, a creature now wholly ruined and defeated, yet still filled with a hideous lust ang rage; and before it stood stern, utouchable by pity, a figure robed in white, but at its breast it held a wheel of fire. Out of the fire there spoke a commanding voice.
‘Begone, and trouble me no more! If you touch me ever again, you shall be cast yourself into the Fire of Doom.’
[/QUOTE]
And a few pages later, Gollum touches Frodo again, and bingo! into the Fire of Doom.
I believe this demonstrates the essential short-sightedness of Evil. The Ring enjoys betraying its non-Sauron bearers, plus it is invoked by Frodo to cast Gollum into the Fire if he ever touches Frodo again. So it gets to betray and obey simultaneously, in a truly disastrous move for its new bearer, Gollum. This is the Ring’s idea of great fun. Unfortunately, it is also the Ring’s destruction; oops.
As the Professors says, “Oft evil will shall evil mar.” The Ring’s evil will did a whole lot of marring that day, because it took the quickie route to gratification rather than waiting for Sauron to come roaring down on Orodruin and taking the Ring from whoever managed to hold onto it, which would have in the long run been the Ring’s best option. But the Ring couldn’t be arsed to wait; it got its jollies where and when it could. Innate problem with Evil, you see.
That’s my theory, anyway. And perhaps only the Ring itself could have the will to destroy it, not counting the Valar.