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The tower of Orthanc is said to have been impenetrable:

By the next day, only the tower of Orthanc remained, as it was hewn of one piece of impenetrable rock...
-"Orthanc", The One Wiki to Rule Them All.

It survives an attack on it by the Ents and has seemingly lasted many thousands of years; it was built in the Second Age.

It is also said that even though it was the Númenórean men who built the tower, it was as though something else was involved:

it was fashioned by the builders of old who smoothed the Ring of Isengard, and yet it seemed a thing not of the craft of Men, but riven from the bones of the earth in the ancient torment of the hills.
-"Orthanc", The One Wiki to Rule Them All.

What makes the tower so impenetrable?

DavidW
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Möoz
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    It is made out of the same stuff they make black box flight recorders out of... ;) – Lexible Apr 12 '15 at 23:17
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    If the rock was impenetrable, how the crap did they hew it in the first place? – user2357112 Apr 13 '15 at 05:47
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    @user2357112 It's impossible for anything to be impenetrable, of course. They just forgot the technology needed to work the material, so it's effectively impenetrable by the time of the Lord of the Rings. Sauron/Saruman/Gandalf might have been able to do something about it, but we simply don't know, and neither did the characters talking about the "impenetrability". – Luaan Apr 13 '15 at 08:23
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    Because of magic. :-) – Martin Apr 13 '15 at 12:05
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    @Luaan so Orthank comes from Dark Age of Technology? – Petersaber Jun 17 '16 at 06:52
  • Because Numenor. – Mark Olson Dec 29 '20 at 00:55

3 Answers3

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The implication is that it's made of some kind of heavy volcanic rock;

For the main wall of the City was of great height and marvellous thickness, built ere the power and craft of Númenor waned in exile; and its outward face was like to the Tower of Orthanc, hard and dark and smooth, unconquerable by steel or fire, unbreakable except by some convulsion that would rend the very earth on which it stood.
- Return of the King

and

They came now to the foot of Orthanc. It was black, and the rock gleamed as if it were wet. The many faces of the stone had sharp edges as though they had been newly chiselled. A few scorings, and small flake-like splinters near the base, were all the marks that it bore of the fury of the Ents.
- The Two Towers

As far as the movies are concerned, according to the LotR "Making of" Appendices, the production crew proceeded on the assumption that the tower was comprised of obsidian, a substance that has long been considered (however erroneously) to be unbreakable:

According to Richard Taylor, the painting of Alan Lee made it look as if the tower walls were chipped out of black obsidian (a kind of volcanic glass). In the FOTR Appendices he explains how the miniature technicians of Weta built up their model by carving every single element out of micro-crystalline wax. Eventually, the 13 feet high movie tower was cast into resin with the aid of a massive silicon mould.

DavidW
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Valorum
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    Although this theory would seem to fit the description perfectly, there's a problem. Obsidian is hardly indestructible - in fact, it's very brittle. – Bob Tway Apr 13 '15 at 08:40
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    @MattThrower But Tolkien may not have known (or cared) about that. – Möoz Apr 13 '15 at 08:56
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    @Mooz oh, quite possibly. Just illustrating that the answer doesn't quite cover the OP's question, in the sense that saying it's made of obsidian fails to explain why it was indestructible. – Bob Tway Apr 13 '15 at 09:01
  • @mattthrower - Obsidian is (historically) considered to be nearly indestructible... – Valorum Apr 13 '15 at 11:48
  • From the wiki: "A peak and isle of rock it was, black and gleaming hard: four mighty piers of many-sided stone were welded into one" by Tolkien. It seems there may have also been some sort of enchantment. – Sarima Apr 13 '15 at 12:05
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    @Richard - Obsidian was chipped to make tool points and sharp weapon edges from the stone age. No one would consider it indestructible. – Oldcat Apr 13 '15 at 21:47
  • @Oldcat - The recent use of obsidian as a semi-indestructible substance in the Minecraft game would suggest that it's still considered to be exceptionally strong. – Valorum Apr 13 '15 at 22:07
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    @Oldcat obsidian is vulnerable to stresses in some directions, very strong in other directions. If you could make blocks large enough, oriented just so, you might make something that would be very hard to damage. Mind I don't think obsidian was meant by Tolkien, he'd have known there aren't blocks large enough for it to ever be used to build a tower the size of Orthanc. More likely he was intending some basalt like rock or maybe marble, and the gleam hints at magical interference holding it together. – jwenting Apr 14 '15 at 05:35
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It's indestructible because of magic.

From The Two Towers, "Flotsam and Jetsam":

"Many of the Ents were hurling themselves against the Orthanc-rock; but that defeated them. It is very smooth and hard. Some wizardry is in it, perhaps, older and stronger than Saruman's"

It was built by the people of the Second Age who had access to more powerful magic than they do in the time of LotR. If they could enchant palantíri to communicate instantly over vast distances, then why not a tower to be really strong?

As a side note, it is strong for the same reason the walls of Gondor are so strong. I don't have a quote for this but they are nigh indestructible.

From The Return of the King, "The Siege of Gondor":

For the main wall of the City was of great height and marvellous thickness, built ere the power and craft of Númenor waned in exile; and its outward face was like to the Tower of Orthanc, hard and dark and smooth, unconquerable by steel or fire, unbreakable except by some convulsion that would rend the very earth on which it stood.

DavidW
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Code Whisperer
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  • Where is your evidence that it was built with magic? – curiousdannii Apr 13 '15 at 15:52
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    @curiousdannii - Two Towers, Flotsam and Jetsam: "Many of the Ents were hurling themselves against the Orthanc-rock; but that defeated them. It is very smooth and hard. Some wizardry is in it, perhaps, older and stronger than Saruman's". –  Apr 13 '15 at 20:11
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    Re: Walls of Gondor, RotK, The Siege of Gondor: "For the main wall of the City was of great height and marvellous thickness, built ere the power and craft of Númenor waned in exile; and its outward face was like to the Tower of Orthanc, hard and dark and smooth, unconquerable by steel or fire, unbreakable except by some convulsion that would rend the very earth on which it stood." –  Apr 13 '15 at 20:16
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    @itcouldevenbeaboat - feel free to add these quotes to your answer if you wish. –  Apr 13 '15 at 20:17
  • @DarthMelkor That's an in-world speculation by a character, but it's better than nothing I guess. – curiousdannii Apr 13 '15 at 21:39
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    Orthanc was made by the men of Numenor, the Palantiri were made by the Noldor. –  Jun 16 '16 at 10:47
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I've read before that the origins are volcanic, though the only reference I could quickly find is from a game:

The impenetrable tower of Orthanc forms the core of the defenses, with a vast circular wall one mile in diameter providing the first line of security. The earth itself formed the citadel, for the outer ring is a volcanic cone shaped by the Men of Númenor with the help of Hill-giants and the spire of Orthanc is in fact a pillar of glossy black volcanic rock known as laen.

In which case the hard part of the Númenórean work was cutting rooms into it.

DavidW
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Wayne
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  • If they could cut into it then how was it impenetrable? – curiousdannii Apr 13 '15 at 01:30
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    By the time of LotR books/movies; the knowledge of how it was formed was lost. The books only state that they were built by the men of Númenor and that nothing left a mark on them. Even the Ents wouldn't have been able to take them down. – Jim2B Apr 13 '15 at 01:34
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    @curiousdannii: It couldn't literally be impenetrable, of course. My thinking is that the effort required by the Numenoreans to modify it was less than creating it from scratch, but more than the Ents, etc, were capable of. Who knows, it could have taken centuries for them to bore some rooms, put in an elevator, a latte machine, etc. – Wayne Apr 13 '15 at 02:10