How do you destroy a Palantír?

Question

Apparently, the Palantíri - just like the most magical artifacts in the Lord of the Rings - were quite durable. One of them survived a long fall from Orthanc, and broke the staircase it hit. But I remember Gandalf saying somewhere that the lost Palantíri might be broken.

What would be my best chance to destroy one of them? Would I need magic powers or a specific location (like the Mount Doom for the Ring) to shatter it, or a huge physical force would do the job? Could the Palantír be made unusable without physically destroying it?

Answer 1

Immense heat, like Orodruin

This is briefly discussed in Unfinished Tales; much like the One Ring, there was no power known to man that could physically harm the palantíri; it was believed that only heat (on the order of Mount Doom) was capable of destroying them:

They were very heavy but per­fectly smooth, and would suffer no damage if by accident or malice they were unseated and rolled off their tables. They were indeed unbreakable by any violence then controlled by men though some believed that great heat, such as that of Orodruin, might shatter them, and surmised that this had been the fate of the Ithil-stone in the fall of Barad-dûr.

^{Unfinished Tales Part IV Chapter III: "The Palantíri"}

Dragon-fire, or at least fire from a particularly old dragon (of which none exist in Middle-earth by the Third Age) could accomplish the same.

That said, I do find it worthy of note that Tolkien uses the phrase "some believed", indicating that this was in-universe speculation rather than the narrator stating a fact. But it's all we have to go on, so there you go.

Could it be rendered unusable without violence?

This is actually substantially easier; in fact there are three known ways you could accomplish this:

1. Take it over the Sea (or generally beyond the reach of Men), which is what happens to the Stone of Elostirion; other stones (specifically, the stones of Amon Sûl, Annúminas, and Osgiliath) were put beyond the reach of Men by being buried at sea; you could add the Ithil-stone to this list as well, if the lore of men is wrong and Mount Doom actually couldn't destroy it.

   You could presumably still communicate with these stones by use of another stone, but you probably wouldn't get anything interesting out of them and you certainly couldn't actually look through them yourself

2. You could render two stones unusable by using them to communicate, and simply keeping the connection open; it's said that only the Osgiliath stone (which was lost at sea) had the power to spy on two Stones communicating:

   Sauron could not break in on these confer­ences: only the surveyor using the Master Stone of Osgiliath could "eavesdrop." While two of the other Stones were in re­sponse, the third would find them both blank.

   ^{Unfinished Tales Part IV Chapter III: "The Palantíri"}

3. Do...whatever it is Denethor did. I've discussed previously that the Stone of Minas Tirith was rendered unusable (except by a person of great strength of will, like Aragorn) following Denethor's suicide. It's never made entirely clear what actually caused this effect, but it's the only indication given of a Stone being "broken" in this way

Answer 2

Could the Palantír be made unusable without physically destroying it?

Sure. In fact, by the time of the War of the Ring, most of the seven palantiri that were ever on Middle-Earth are unusable for one reason or another. Two of them went down with the ship of the last king of Arnor in the year 1975 of the Third Age:

1975 Arvedui drowned in the Bay of Forochel. The palantíri of Annúminas and Amon Sûl are lost.

_{-- Lord of the Rings, Appendix A}

The stone of Osgiliath was lost during the sack of that city in the year 1437. It may have fallen into the Anduin, or been physically destroyed; in any case, it is unusable:

1437 Burning of Osgiliath and loss of the palantír.

_{-- Lord of the Rings, Appendix A}

Tolkien Gateway (which is unusual for a wiki site in being very reliable) has a long section on this:

One by one the stones vanished from public knowledge or were lost. The Osgiliath-stone fell into Anduin during the Kin-strife and burning of that city in T.A. 1437. When Arvedui, King of Arnor, was shipwrecked and his line ended in 1975, he drowned with the palantíri of Amon Sûl and Annúminas, the only communicating stones of Arnor. When Minas Ithil fell in 2002, the stone was assumed destroyed in general. The wiser and more foresighted men of Gondor decided that in case Sauron had seized the Stone, they would stop using the Anor-stone to prevent any contact with the Dark Lord. As the Elostirion-stone was locked away and could not answer the other stones anyway, the only remaining stone was the Orthanc-stone, which became useless to the Gondorians. [...]

The final fate of most of the stones is unknown. The Elostirion-stone was taken west with the Ring-bearers in 3021 of the Third Age, severing the last link of Middle-earth to Valinor. The stones of Anor and Orthanc are believed to have been reinstated in the Reunited Kingdom and used officially once more. The Ithil-stone may have been destroyed in the fall of Barad-dûr, but it is also possible that it too was found and reused in the Reunited Kingdom. Whether or not the other three lost stones were ever found is never indicated; the Osgiliath-stone may have rolled into the Sea, or it may have lain still in the Anduin. The stones of Arnor, however, were lost in the frozen seas of Forochel, and therefore it is highly unlikely that they could ever be recovered.

_{(emphasis mine)}