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Would it be possible, with genetic engineering, to create a form of tree which produces wood with the same strength/hardness as granite, capable of being used in place of stone for castle-style walls and other defense structures? What minerals and/or nutrients would be required?

This plant would not need to be able to survive in the wild, nor will it need to evolve, so don't worry about that level of plausibility. It is ok for the plant to not be self-sustaining, that is it fine for it to require added fertilizer and food to grow. The group creating these plants has access to technology for genetic engineering much greater than Earth's, but otherwise has late-medieval/early renaissance technology.

Minor deviations in the way plants work are fine.

Pure rock is not acceptable, the resulting wood must still be at least technically wood, with some organic material. Essentially, the product must be distinguishable from ordinary inorganic rock when a small bit of it is analyzed in a modern lab without context. I'm OK with a plant that basically creates granite or a similar rock with cellular residue left in the rock, but I would like an explanation on how this plant could grow with this world's physics. How does a plant avoid turning all of its living cells into rock, and still grow to a sizable height?

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    $\begingroup$ Animals make bones, why not let your trees actually make stone? There isn't any physical reason why not. You just need the right fertilizer $\endgroup$ – Raditz_35 May 5 '17 at 12:54
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    $\begingroup$ I altered your title because wood as hard as soapstone or chalk is not a real challenge ;) $\endgroup$ – Mołot May 5 '17 at 12:55
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    $\begingroup$ So what is your reason against making it actual granite? Magic can't modify it if it's made out of lignin? So have it be 99% granite and 1% cellular residues, just enough to make it qualify as wood by the rules of your magic. The cellular residues could even add to the stability $\endgroup$ – Raditz_35 May 5 '17 at 13:09
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    $\begingroup$ This basically exists in nature today. Fossilized wood is what we call it, but I think the most likely process you need to read about is perimineralization. It's not really a function of the plant, but it would accomplish what you want without handwavium. $\endgroup$ – UnhandledExcepSean May 5 '17 at 14:55
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    $\begingroup$ Not sure if they work but I see ads for pills to do this all the time... $\endgroup$ – R.. May 5 '17 at 17:40
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Nature already got your back, buddy.

Schinopsis brasiliensis is a extremelly hard wood native from northern Brazil. It's main use is on the construction business as a reinforcement for the structure of the roof and for furniture, support for train tracks, or even as a replacement for steel reinforcement while concreting stuff. Oh, and bludgeoning weapons. Nice, really nice bludgeoning weapons.

This wood is so hard that that it was nicknamed "quebraxo", or "quebra-machado", meaning axe-breaker. It is a bit ligther than granite, having around 1.5g/cm³ for its density and being twice as hard as bubinga wood.

This wood is so hard that punching a concrete wall is actually less painful than punching one of those evil trees.

(I may be a bit paranoid but I think they are actually made of alien micro-nanites ready to inject mind-controlling drugs on the populace. But don't let them know that I told you so.)

In fact, regular tools, like axes and handsaws (saws you use with your hands, not to cut hands. You get it.) have a really hard time to cut it. You need a very good chainsaw or a water-cooled industrial saw to cut trough a bunch of those without losing your tools.

Luckily, those trees grow in really useful sizes, being mostly straight up. This means that you can put them to use with less cuts, which makes them a way easier to use material than granite while losing to it by very little regarding durability.

Australia has a bit more extreme wood from the aptly-named ironwood family, which beats even the Schinopsis and is probably the closest thing you can get to a rock that makes photosynthesis. And is also probably poisonous. And carnivorous. And infested of snake-eating spiders and who knows what more. Australian things are scary.

Australia is scary.

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    $\begingroup$ I wish I could upvote twice... $\endgroup$ – WRX May 5 '17 at 21:09
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    $\begingroup$ Ahh, Australia. You suggest the wood is poisonous and carnivorous, and instead of asking for sources to back your claim, I sigh and say "Man, Australian plants are brutal!" $\endgroup$ – Cort Ammon May 5 '17 at 22:05
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    $\begingroup$ I can tell you that the Australian timber Mulga is indeed poisonous, splinters can kill you if not properly treated. As for hardness, well, let's just say I sand Mulga with angle grinder disks meant for steel... $\endgroup$ – Samwise May 6 '17 at 0:04
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    $\begingroup$ @VilleNiemi We use them a lot here on my country for all sort of stuff. We have some really awesome wood houses made of those are sturdier than the regular concrete+bricks houses we have around. In some areas this type of tree is considered endangered on the wild, so most of the comercial ones come from reforestation areas. $\endgroup$ – T. Sar May 6 '17 at 13:46
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    $\begingroup$ @Catalyst Wait....does Australia actually have a fire swamp? Because if it does I want to know. $\endgroup$ – kingledion May 7 '17 at 22:41
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Compression strength of granite is 19 000 psi. Compression strength of bubinga wood is 10 500 psi. So we are more than half way there when it comes to building high walls. Great! Actually even better, as bubinga is $0.71 g/cm^3$ and granite is between $2.65$ and $2.75 g/cm^3$ so your walls could be higher.

Bubinga's hardness is 2 690 lb and bending strength about 22 600 psi (for granite, bending strength seems up to about 7500 psi) - Whilst I can't make full calculations and analysis, I believe walls made of it could be more resistant to shatter, less likely to splinter than granite. If you could find a big enough supply, you could make really decent walls out of it.

All in all - surprisingly we are not that far from what you ask already!

If you want to go even harder, you need to let it deposit even more minerals in it's structure, grow even denser.

Or you could let the wood sit in the swamp, the way Polish black oak is created (kind of bog-wood, automatically translated Polish article here). A lot of tannins and a lot of iron in water, creating really hard structures inside the wood. But it works best on trees that fallen into the water and died. That said, it wouldn't be unbelievable in the heartwood of live tree. Given that the process causes our black oak to get up to $1.1g/cm^3$, and the difference is actually very much stone-like mineral, if you will apply this to bubinga, you should have best of both worlds.

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    $\begingroup$ @DonyorM Best pieces are about 500 years old, but that's with only unaided diffusion of iron compounds. If your tree will pump it actively, it could probably happen in "real time", with yearly rings hardening one per year, at least in younger trees. $\endgroup$ – Mołot May 5 '17 at 14:11
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    $\begingroup$ By "the best of both worlds", is there any chance you mean "best of both woods"? $\endgroup$ – John Dvorak May 5 '17 at 15:20
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    $\begingroup$ You can use one of my country's trees, the Schinopsis brasiliensis, instead. It overkills the bubinga with the around double of it's specific hardness, nearing 5k/lbf! This tree's density nears 1.5g/cm³, being way closer to granite, almost rock-like naturally. This tree is actually heavier and sturdier than the black oak by itself! $\endgroup$ – T. Sar May 5 '17 at 17:32
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    $\begingroup$ @TSar this would make good answer on its own. Have you posted it already? Looking for it to upvote if you did $\endgroup$ – Mołot May 5 '17 at 20:18
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    $\begingroup$ Re: "the way Polish black oak is created (sorry, can't see English text about it)": The English term seems to be bog-wood. $\endgroup$ – ruakh May 5 '17 at 23:01
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Yes, you could get wood harder than granite

What you're going to need

  1. Enzymes for creating diamonds
  2. Enzymes for making graphene
  3. Genome for the Nuttal Oak since it's a fast-growing hardwood tree

Diamonds are amazing at compression but are outperformed by other common materials in tension. Graphene has the highest tensile strength of any known material but essentially no compression strength since it's a sheet. Let's make a hybrid tree that utilizes both these forms of carbon.

In wood, cellulose makes up the walls of each cell with pectin and other binders between cells. Replace those intercellular binders with graphene and replace the cellulose walls with a porous diamond matrix. (It needs to be porous so that cell can still breath.)

As the tree grows, it will build cell walls out of diamond then connect to the cellular neighbors with graphene sheets. If small spikes are put on the diamond cell walls, when under extreme loads, those spikes will pop into the cell pores for extra grip strength.

Complications

  • Working with this diamond wood is going to be extremely difficult. Consider placing the tree in a form so it grows into the shape you want.
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    $\begingroup$ That's pretty awesome. I may use a combination of different posts, and have different levels of tree wood with different costs. $\endgroup$ – DonyorM May 5 '17 at 14:58
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    $\begingroup$ Do enzymes for making diamonds and graphene at STP exist? $\endgroup$ – BobTheAverage May 5 '17 at 20:26
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    $\begingroup$ Bear in mind that often it's only the outside part of the tree trunk that is 'alive', so the inner part could be a pure diamond/graphene composite. I strongly suspect that such a tree could grow to the edge of space, at least physically (water transport would be the biggest issue). $\endgroup$ – Andrew Dodds May 8 '17 at 8:29
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    $\begingroup$ ...And then the fantasy answer. Glorious diamond trees. I doubt you could actually create an enzyme that would create diamond latices (graphene I'd believe) as it takes very high pressure for carbon to take the proper arrangement. Even at room temperature and pressures the only process I see involves cavitation bubbles which just cause localized extremes (and results in very tiny diamonds). $\endgroup$ – Draco18s May 8 '17 at 18:05
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    $\begingroup$ @Green Oh gosh no, don't retract your answer. Diamond tress sound delightfully fanciful. Its just that your answer prompted me to figure out what the hard-science bounds were on it and being unable to find that information and having to ask. I simply shared the answer I got. :) $\endgroup$ – Draco18s May 8 '17 at 19:46
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Petrified wood may fit these requirements: While it is, compositionally, just rock, the way in which it is formed makes it very different from quarried stone.

Petrified wood is formed from wood that dies in certain environments with mineral-rich water and is rapidly covered by soil or ash. The minerals inhibit decomposition, and gradually deposit within the tissue of the wood. Over time, as the original wood decomposes, it is replaced with mineral deposits that assume the same shape and structure of the wood. The end result is rock, but rock that is structurally identical to the wood.

In the real world, petrified wood is often found in volcanic ash and other niche environments, and takes decades to centuries to form naturally. However, about ten years ago a team of scientists demonstrated a technique whereby wood was artificially petrified in a matter of days.

In a sci-fi context, you can take this basic idea and alter it slightly: Your people build structures out of plant matter, using genetically engineered fast-growing strains and artificial growth techniques to speed up the process, and mold the plants into the structure they need. Once the correct shape is set, they use an artificial petrification solution (perhaps a spray or water additive) to arrest growth and start the petrification process. Once the wood is fully petrified, you have a stone structure in the carefully-grown organic shape of the original wood.

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  • $\begingroup$ If I chipped off a tiny bit of this petrified wood and brought it to a modern lab, could they -- without context -- tell me if the rock was completely inorganic or from petrified wood? $\endgroup$ – DonyorM May 5 '17 at 16:01
  • $\begingroup$ Nice. Similar to Polish black oak, but with oak there is little to no decomposition of original wood, just addition of rock. Glad to see you can go even more "rocky" . $\endgroup$ – Mołot May 5 '17 at 16:02
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    $\begingroup$ @DonyorM, it is almost entirely inorganic compounds once the petrification process is complete, but it copies the vascular structure of the original plant. It would be very obvious under a microscope that it is essentially a tree made of rock, and therefore neither natural rock nor a natural plant. $\endgroup$ – Catgut May 5 '17 at 16:07
  • $\begingroup$ +1. @DonyorM it seems like your key requirements are that future archaeologists would be able to see that the structure involved generically-modified plants and the odd strengthening of wood? The GM part of this would not be that the wood was stone-strong -- that's due to advanced chemistry -- but that the trees grew into building shapes and in a short-enough period of time to be useful, and so couldn't be natural. $\endgroup$ – Wayne May 6 '17 at 23:27
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Of course it would be possible.

There are many cases of biological organisms creating inorganic structures that are often much better than anything purely inorganic. Spider silk comes into mind, as it is much stronger than steel for it's weight. Or in your case, something like process, that creates pearls.

But there are two problems : resources and speed.

The resources problem is simply the organism having access to enough raw materials to construct the structures and energy to move things around. But that could be solved by "feeding" the organism a soup of raw materials dissolved in energy-rich soup.

Second is speed. As seen with the pearls example, biological organisms are often awfully slow at building those inorganic structures. I don't know how fast it can be made, but if you strip away any biological parts that are not necessary for construction of the inorganic structure, make it's food really easy to digest and add some handwavium, you could grow few millimeters per day.

Your question really reminds me of how synthetic insulin is produced. Scientists took some bacteria, and altered it's genome so it produces insulin. Then, those bacteria are grown in vats filled with raw materials and the insulin is extracted after some time.

Also, I don't think you want rock as the inorganic structure, if you want the biological parts to go through it. What you should be looking for is some kind of porous-foam like material. Possibly made from metal. Like some kind of metal foam. Possibly with holes being microscopic as not being visible to naked eye and being easier to make by biological organisms.

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  • $\begingroup$ I don't really see where this answers the question. I appreciate the examples, but how does this tie up to strong wood? Could you tie it all up in a summary paragraph? $\endgroup$ – DonyorM May 5 '17 at 14:06
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Well, based on high school biology from 28 years ago, and what I was awake for, Isn't a substantial amount of what plants are made from come from the soil?

So what you do is you have your genetically or magically altered plants incorporate Granite or maybe limestone or basalt from volcanic rock into the cell wall structure?

Volcanic rock and soil are great for growing Bonsai plants in. The plants take in the soil in greater proportion and form a basalt formation in the cell walls to create a wood that has a lot of the same properties as Basalt.

LImestone is even easier. Suppose it takes in CO2, uses part of it for photosynthesis and uses the rest of the carbon to form CaCO3, or Calcium Carbonate to use in the cell wall structure.

I'm not so sure about granite since it is a heat and pressure kind of igneous rock. I cant think of how to hand wave the required amounts of mica, feldspar, and quartz.

Here is why I think having the plant incorporate the substance into the cell walls is important. If you have ever looked through a microscope at some onion, it kinda looks like a brick wall. Onions grow in layers. something else reminds me of a brick wall and has layers: Timbrel Vaulting. Timbrel vaulting is very strong for it's mass and thickness.

Combine drawing minerals from the soil and incorporating them into cell walls with a plausible structure and you get plants as strong, if not stronger, than stone.

I think growing stone could be fun

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  • $\begingroup$ I'm not worried about the type of stone as much as the strength. Would your suggestions have the strength of a granite-like rock? $\endgroup$ – DonyorM May 5 '17 at 13:51
  • $\begingroup$ They could. I edited to add some notes about onions and timbrel vaulting. That gives some ideas about how the stone like substance can be combined with a plausible structure that should yield an incredibly strong material. Follow the link and you will see what I mean $\endgroup$ – Paul TIKI May 5 '17 at 14:04
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    $\begingroup$ It might be surprising, but very little of what trees are made of comes from the soil. Trees (and other plants) are made mostly out of the stuff they pull from the air- CO2. In fact, trees are about 95% air (or rather, 95% of their "stuff" came from the air). $\endgroup$ – Delioth May 5 '17 at 15:59
  • $\begingroup$ Didn't know that. I guess that makes the Limestone option a little easier. I was mentally more focused on the structure rather than the composition. I find Timbrel Vaulting fascinating. $\endgroup$ – Paul TIKI May 5 '17 at 16:03
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Silica in crystalline form is actually not uncommon in plants (see https://en.wikipedia.org/wiki/Phytolith). Silica IS quartz. Quartz (Mohs hardness 7) is also one of the two main hard constituents of granite, feldspar (Mohs hardness around 6) being the other. This is harder than unhardened steel (common construction steel is not hardened, tool steel is).

There might not even be a need for high tech genetic engineering (as opposed to simple and patient selective breeding) to achieve very hard plants - see the examples of real life ultra-hard woods in other answer...

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This may be a bit farther out than you were seeking, but I direct you to Pyura Chilensis

This is not a plant, but it is feasible that a more plant-like organism could similarly form a hard rock-like shell from mineral debris in its environment.

This is a filter feeder that looks like a chunk of rock with internal organs; I cannot easily find information on the hardness / composition of that shell, but it looks to be potentially similar to that of oysters or barnacles.

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  • $\begingroup$ Thanks for answering! Could you pull the main elements from your link into your answer, so someone doesn't have to follow the link to get the gist of the answer? And would this shell be able to be distinguished from ordinary inorganic rock if analyzed in a lab without context? $\endgroup$ – DonyorM May 5 '17 at 19:47
  • $\begingroup$ That link is a little scarce on detail too. Are they much tougher or bigger than coral or muscles? $\endgroup$ – user25818 May 5 '17 at 20:01
  • $\begingroup$ No idea. They are not well understood creatures from the look of things. I can't even find any references to how hard their shell would be; ie. do they just "look like rocks" or is that shell indeed hard and not sponge-like. But it looks like it would be comparable to oyster / barnacle shells. I also cannot tell if the shell is wholly generated by the creature as it grows, or if it is instead a composite of sand and debris. $\endgroup$ – FunkThompson May 5 '17 at 20:03

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