With the discovery (in the early twentieth century) that molecular biology is based on the interactions of carbon-chain molecules, it was naturally suggested that extraterrestrial life would probably also be carbon based, but it might be based on silicon instead—silicon being the most similar element to carbon, chemically. The feature that makes carbon powerful as a base is that it tends to form four covalent bonds. Silicon behaves similarly, since it lies directly below carbon on the periodic table. However, in practice, silicon has disadvantages. It is about seven times less abundant than carbon, but more importantly, the silicon atom is larger (2.1 Å, versus 1.7 Å), which makes the bonds it forms longer, and thus weaker, and thus easier to disrupt. However, it is by no means inconceivable that silicon-based life could exist in the universe.

In science fiction, however, silicon-based life is often portrayed quite differently. The association of silicon with rocks on Earth leads to depictions of aliens that are distinctly rock-like, such as this fellow from the Doctor Who story "The Hand of Fear."

Eldrad must live.

Even worse misunderstandings of what silicon-based life would mean occur in another Doctor Who serial, "The Stones of Blood" (where the supposedly silicon-based organisms are dependent on carbon-based amino acids); or The X-Files episode "Firewalker," again with silicon-based life somehow involving carbon-based chemistry; or any of numerous other examples.

Having silicon-based life forms look like rocks is, chemically speaking, roughly equivalent to expecting carbon-based life (e.g., us) to look as if it were made out of graphite and/or diamond. However, this seems a very common misconception. My question is: Where did this idea—that silicon-based life would have rock-like properties—originate?

The earliest example that immediately springs to mind is Jack Williamson's short novel After World's End, from 1939, but in that work, the idea of rock-like organisms seems like it was already well established. (There are also other oddities to the silicon-based organism in that particular story that suggest it was probably not the first to introduce the idea.) So where and when did the idea originate?

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    Camille Flammarion's Lumen (1873) has "On one of the planets of the system of Aldebaran, very curious from this point of view, the vegetables are all composed of a substance analogous to the loadstone, because silica and magnesia predominate in its constitution. The animals feed on this substance only. Most of the beings inhabiting this world are incombustible." (source PG) But this doesn't actually describe the animals as "rocky." – DavidW Dec 22 '19 at 22:49
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    Well, carbon is known for coal, graphite, and diamonds, so carbon-based life forms should be lumpy black people with sparkly eyes that are very slippery... – Harper - Reinstate Monica Dec 23 '19 at 6:59
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    Coal comes from carbon-based organisms, so it won't be familiar to silicon-based science-fiction writers. – user3757614 Dec 23 '19 at 7:32
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    Could it be that the idea of rocky monsters arose first, and the silicon was used later as a convenient-sounding explanation? (we know that carbon-based life tends to be woody or squishy, so those rocky trolls must be based on something else...) Scandinavian trolls were sometimes described as turning to stone when exposed to sunlight, and some 20th century authors referred to this as a reversion to the rocks the trolls were made of. – Pastychomper thanks Monica Dec 23 '19 at 16:19
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    @DavyMwenttofundMonica But we're made of meat. – Dai Dec 24 '19 at 3:10

It seems that silicon life forms in old science fiction are almost invariably described as rocky or crystalline. With the help of Science-Fiction: The Gernsback Years by Everett F. Bleiler I found four examples antedating my previous answer, "Out Around Rigel".

1931: "The Planetoid of Peril", a novelette by Paul Ernst, first published in Astounding Stories, November 1931, available at the Internet Archive.

There was that matter of the stone splinter, however, which certainly argued that the wan, prematurely white-haired fellow was a little unbalanced, and hence not to be believed too implicitly. He'd handed it to Harley, and gravely declared it to be a bit of the monster's flesh.

"Why, it's only a piece of rock!" Harley had exclaimed before he could check himself.

"Did you ever see rock like it before?"

Turning it over in his hands, Harley had been forced to admit that he never had. It was of the texture and roughness of granite, but more heavily shot with quartz, or tridymite than any other granite he'd ever seen. It had a dull opalescent sheen, too. But it was rock, all right.

"It's a piece of the thing's hide," the man had told him. "It flaked off when it tried to pry open the manhole cover of my Dart. A moment after that I got Radivision arc directions from Landon Field, aimed my sights, and shot for Earth. It was a miracle I escsped."

1931: "The Arrhenius Horror", a short story by P. Schuyler Miller, first published in Amazing Stories, September 1931, available at the Internet Archive.

"'God!' cried Tom in a stifled voice. 'The energy of it! Bill, man, have you ever seen the like? Bill, it can't be earthly — it's too huge, too colossal for earth. It would drain our little planet of energy and life, and leave it an empty husk in Space! It must be from beyond, from out there among the stars and galaxies. There are worlds out there that no man may imagine, or even dream of.'

"'Tom!' I shouted. "Tom, can't you see? It's moving! It’s growing! Tom, that Thing is alive!'

"All the great tumbled heap of crystal was stirring slowly, uneasily, with life. Huge hexagonal prisms were swelling visibly, rising toward the mist above. Now a broad facet burst with a shattering clang, and a slender triangular shaft shot from its face, out and diagonally up, blue glory streaming from its tip. Now a thin rectangular column was thrusting up and up, with uncanny speed, from the very summit of the pile. And now a second shaft was darting toward it from one side, catching it square, bringing it down with a clash of crystal on crystal, even as new and different forms sprang from its shattered base. And as it grew upward, the great mass spread outward, toward us, with that steady creeping that gives the impression of awful relentless speed and purpose. Often, in years gone, I had dropped crystals in a beaker of water-glass, sodium silicate solution, and watched them swell and send up pseudopods in a weird semblance of life, forming a strange submarine garden of sponge and sea-fern and tentacled polyp. But they were but phantoms of solidity — fragile shells filled with dense solutions, while these things, growing with the same strange speed, were solid throughout. In my mock-garden, the water-glass had fed them the silica that made them grow, but here — I did not know. It must suffice that they were growing and spreading from some hidden central source, with the twang and clash of striking crystal, and the high hum that told us that the whole great body of the thing was vibrant with energy, and with something akin to life.

1930: White Lily, a novel by John Taine (pseudonym of the mathematician Eric Temple Bell), first published in Amazing Stories Quarterly, Winter 1930, available at the Internet Archive. A substantially rewritten version was published in 1952 as The Crystal Horde.

First one bubble survived, then another, until the whole desolation became a city of bleak, colossal domes. The domes began to glitter icily in the moonlight, and almost instantly the imprisoned life that was in them burst forth and multiplied. Jostling one another to ruin, the furiously increasing masses of crystal flashed out gleaming arms that branched and begot new colonies of glittering crystal; these fed for a moment on their generative substance, then instantly burst out in explosive growth in all directions, repeating the conquest of space. As yet they had no mode of locomotion; their sole power over distance was growth. In their urgency to survive, the opposing masses shot their gleaming progeny at one another, devouring the diminishing distances between them.

Their one instinct, if they had any, was to exterminate their competitors by seizing and absorbing all of the food by which they might increase.

Before the last alley closed and became a dense mass of furiously growing crystals like the rest, the mountain range that had flowed over the plain was a vast concourse of gigantic crystal shapes, towering and flashing in the moonlight, that shot upward with an accelerated growth that menaced the sky. Each glittering pinnacle budded at a thousand spax-kling points into living spears of crystal; these shot into the night, themselves to become the sources of explosive life. Neither upward nor lateral growth apparently had any natural limit, and the huger the vast bulks became the faster they grew. The whole mass pulsed and flickered with striæ of green and purple light deep within the hollow crystal masses, and from the brittle, spearlike points streamed steadily upward innumerable brushes of clear, cold blue light. The loftiest pinnacles, leaping skyward with an ever greater speed, bristled with electricity whose sharp, dry hiss all but drowned in volume the creeping rustle of an incessant growth.

1929: "The Gas-Weed", a short story by Stanton A. Coblentz, first published in Amazing Stories, May 1929, available at the Internet Archive.

It was the massed attack of several thousand men that brought the greatest surprise of the day. The plant-like growth, though apparently soft and flexible, proved to be actually hard and impermeable as granite! Again and again the attacking bayonets struck with a clatter as of iron against rock; again and again the blades were warped or broken. And the hand grenades and the aerial bombs exploded without causing any visible damage; the shells of the field artillery made only narrow gaps, which closed almost instantly, leaving all as before! The supposed plants were really stronger than steel!

[. . . .]

Within a few days, a number of other extraordinary facts about the gas-weed had been made public by Krass. For one thing, he had conducted a chemical analysis of the young shoots, when they were still relatively soft and tender and had not attained anything of that steel-like rigidity characteristic of the developed plants. He had discovered that, in common with the seeds, they were composed of a silicon compound, so highly complex that its chemical formula defied analysis, and differed fundamentally from any other substance ever known on earth. Krass's theory — based, it is true, upon incomplete researches, but later thoroughly substantiated — was that the gas-weed had a totally different chemistry from any terrestrial organism: instead of the chlorophyl common to all green plants, it had a reddish pigment which enabled it to utilize the sunlight as a source of growth; while, in place of the well-known protoplasm, with carbon and nitrogen as its basis, it had a molecular construction equally elaborate, but with silicon as the essential ingredient. This, in Krass's belief, explained not only why the weed throve so well in the sandy soil of the beach, where silicon dioxide existed in inexhaustible quantities, but why it was able to build up walls of the flinty construction for which various silicon compounds are noted. Such are the felspars, trap-rock and others.

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A Martian Odyssey (1934, so a bit older) has a silicon-based lifeform that excretes rocks (or bricks).

One of the characters says "...the creature was rock..."

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    Not to mention it's a landmark classic.... – Spencer Dec 22 '19 at 22:51

1931: "Out Around Rigel", a novelette by Robert H. Wilson, first published in Astounding Stories, December 1931, available at the Internet Archive. Not exactly what you asked for, since the silicon creatures are described as glassy rather than rocky.

A couple of adventurous Lunarians land on a planet of Rigel:

Through a stop-cock in the wall, Garth had drawn in a tube of gas from the atmosphere outside, and was analyzing it with a spectroscope.

"We can go out," he said. "It's unbreathable, but we'll be able to use the space suits. Mostly fluorine. It would eat your lungs out like that!" —

"And the suits?"

"Fortunately, they've been covered with helio-beryllium paint, and the helmet glass is the same stuff. Not even that atmosphere can touch it. I suppose there can be no life on the place. With all this sand, it would have to be based on silicon instead of carbon — and it would have to breathe fluorine!"

Of course there is life, and it's hostile:

And so we stood back to back, hewing out a circle of protection against our enemies. They seemed to have no fear, and in spite of the destruction our blades worked among them, they almost overcame us by sheer numbers and weight. It was a case of whirling our swords back and forth interminably in the midst of their tentacles. Against the light, the long arms were a half-transparent brown. Our swords broke them in bright shivers. Formed from the predominant silicon of the planet, the creatures were living glass!

For perhaps a quarter of an hour we were in the thick of them, hewing until I thought my arms must fall, slashing and tearing at the ones that had got underfoot and were clamping their tentacles around our legs. Only for the space-suits, we should have, by this time, been overpowered and torn into bits — and yet these garments could not be expected to hold indefinitely.

But at last there was a breathing space. The crippled front ranks dragged themselves away, and there was left around us a brief area of sand, covered with coruscating splinters of glass. Garth got the breath to say something or other encouraging. It was like old days at school.

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    Nice find! For these, purposed, I would consider "glass" to be effectively a type of rock. – Buzz Dec 23 '19 at 5:38

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