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In this world, there exist a race of humanoid people which, for simplicity, we'll call Rockfolk.

Their skin is grey, and visually resembles stone. It is also exceptionally tough and durable, to the point where it cannot be pierced by modern firearms, even if such weapons actually could damage an equivalent mass of inert normal stone. Bullets hit their flesh and are flattened on impact or richochet off, without harming the individual in any meaningful way.

On top of this, their flesh is flexible enough for them to still be mobile and live out normal lives in the same fashion as humans would.

Other than this one major feature, Rockfolk resemble humans as closely as is physically possible for a race with such a feature.

The question is: How closely is that?

Assuming their bodies function in such a way that having such skin is helpful, or at least not actively detrimental to their health, what other features or qualities would they necessarily have (or lack) that would differ from a human?

For example, I imagine they would not have any body hair, as it could not pierce through their own skin, and thus they would be vulnerable to cold when exposed.

Note: This is not a question about whether or not having bulletproof skin is possible or plausible. Assuming it is, and this is what it looks like, I'm interested in possible and plausible side effects.

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    $\begingroup$ Minor point, but hair does not pierce the skin. It grows out of pits/sacs in the skin. $\endgroup$ – Frostfyre Feb 28 '18 at 17:49
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    $\begingroup$ "Actively detrimental" in this case would also include the energy expenditure to grow and maintain such skin. You probably also want to consider that just because something stops bullets doesn't necessarily mean it'll stop, for example, a piercing weapon such as a knife or blade (or a set of large teeth). It sounds like it would in your case, and in that case I can think of situations in which it might be beneficial, but there's no law of nature that says it has to be. The mechanics of stopping a bullet and stopping a piercing weapon are quite different. $\endgroup$ – a CVn Feb 28 '18 at 18:27
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    $\begingroup$ Do they shed skin like humans do? I am thinking of dust... $\endgroup$ – user1008090 Feb 28 '18 at 22:21
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    $\begingroup$ could have the same problem brains inside skulls produce if the tissue underneath gets bruised and swells it cant actually swell outwards so instead it ends up cutting off its own blood flow. $\endgroup$ – John Mar 1 '18 at 5:44
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    $\begingroup$ Not that it's exactly the same, but Luke Cage. Basically, it's all fine and well until someone finds a funmetal bullet that does penetrate and they have to do surgery. Goes the same for any other regular maintenance where surgery is necessary, unless you make your creatures have no health issues ever. $\endgroup$ – TC1 Mar 1 '18 at 11:39
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If we look at what skin does we can figure out how changing it will impact things.

  1. Temperature Regulation

The skin regulates our temperature, both in cold and hot conditions. Blood vessels contract and expand to release and retain heat. The included fat also acts as an imperfect thermal barrier.

  1. Creates a sealed environment

The skin keeps out toxins, germs and really anything else that exists in our environment. Yet it is light and flexible enough that we are still pretty agile creatures.

  1. Touch receptors

We feel stuff via nerves in our skin.

  1. Storage System

Or skin stores water and fat and other metabolic products.

What happens when we make our skin bullet-proof?

Well it sort of depends on how you want to do that. You can either make the skin more dense, make it thicker, or since you want to stop bullets, likely a combination of the two.

Thicker, denser skin will make temperature regulation a challenge. Either your rock men should live in a colder climate, that is less prone to temperature swings or you are going to have to hand-wave this. Alternatively you could alter the method of heat reduction, dogs as the obvious example pant to reduce their body temperature. This may still require a bit of hand-waving, I am not sure but I would hazard a guess that panting becomes less effective/efficient as the size of the animal increases.

When you increase the density and thickness of a substance you make it stronger but you also make it more inflexible. Odds are these humanoids of yours are not going to be as agile as a regular human. Conversely they will likely be stronger. As one other post mentioned they are going to need to be stronger to support the additional weight of their armored skin. The additional weight reduces their agility as well...momentum and all that.

I suppose this would also lead to reduced touch sensitivity, which considering the skin is much tougher isn't a huge problem in a lot of ways. It would be a problem when it comes to fine motor skills and you are probably going to want to make the skin thinner where need be to account for that. On a side note, facial expressions are going to be pretty drastically subdued if they exist at all, its tough to emote if your skin doesn't want to move.

As far as the point on storage goes I don't see any problems in the cross over. If anything you would probably want to make this system more robust as getting hit in un-padded armor will still hurt pretty bad.

Final notes:

Keep in mind that if this evolved via natural means you won't have uniform thickness. Having the core and perhaps upper legs and arms armored would provide protection and help maintain agility, plus the resources to armor everything are biologically expensive...best survival wins. When it comes to survival really tough skin is good, but so is mobility. Rhinos and Elephants have super thick skin sure...but they also don't have fingers. Maintaining a decent level of mobility is going to be important for your humanoids, if you want them to in fact be humanoid.

As someone else mentioned you can have hair if you want to, it doesn't technically grow through the skin, but out of little holes.

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    $\begingroup$ Your "final notes" remind me of a story of WWII bombers getting additional armor where the returning bombers' bullet holes weren't - the planes being shot through the wings were making it home; the ones being shot through the cockpit, less so. Rockfolk getting shot through the arm would likely make it home; rockfolk getting shot through the torso or brain-pan, less so. $\endgroup$ – minnmass Feb 28 '18 at 22:51
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    $\begingroup$ Minor point: Blood vessels do not move. They dilate or contract in place. I've never heard someone claim they move rather than change diameter, so I had to look it up, apparently it's a common misconception. Note that blood vessels under a rock skin would still work the same way. It's plausible that the rock skin is thermally conductive, so blood vessels could still transfer more or less heat onto the skin for dissipation. $\endgroup$ – Samuel Feb 28 '18 at 23:14
  • $\begingroup$ Temperature regulation: Who said they have to be endothermic? Sure, that'd be more human-like, but ectotherms might have less of an issue with this. It'd still depend on the climate being agreeable, though. $\endgroup$ – Kevin Mar 1 '18 at 6:30
  • $\begingroup$ @Samuel Another minor point: blood vessels do move, but not closer to or further from the skin for temperature regulation purposes. I can see my blood vessels moving when I flex my fingers. $\endgroup$ – Logan Pickup Mar 1 '18 at 15:06
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    $\begingroup$ @LoganPickup Perhaps saying they don't migrate makes the point better. $\endgroup$ – Samuel Mar 2 '18 at 2:21
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Let's assume their bullet resistance comes from an ability to resist rapid compression (think oobleck) or "shear thickening". This would allow "their flesh is flexible enough for them to still be mobile and live out normal lives".

That being the case, they could still get cut (though not as easily) by a sharp but slow moving blade (think Dune's slow shield). This would drastically change combat styles. Another cool effect would be dealing with space: Rapid decompression due to cabin breach? Not a problem...for a few minutes at least. Physical contact sports would require little if any padding.

If Rockfolk get to interact with other species like humans, I would expect their hides/bodies to be highly sought after for its uses.

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    $\begingroup$ Oobleck, or non-Newtonian fluids, don't resist compression any more than other liquids. They simply have a viscosity that depends on sheer rate. Additionally, rapid decompression is a problem for anything that breathes air. The problem humans have with decompression is the reduced availability of oxygen or with gases in our lungs or dissolved in our blood expanding in low pressure environments. Having temporarily less viscous skin won't change any of that. $\endgroup$ – Samuel Feb 28 '18 at 23:32
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    $\begingroup$ @Samuel The linked Wiki article disagrees with your first point. I have not read the cited note, though. The point on vacuum is correct as far as I can tell. $\endgroup$ – Angelo Fuchs Mar 1 '18 at 14:54
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    $\begingroup$ Decompression hurts you in two ways: 1) If it's fast enough the escaping air will shred your lungs. 2) Lack of oxygen. You have 10-15 seconds before the deoxygenated blood reaches your brain and you pass out, death comes in like 4 minutes as it normally would for no breathing. Your body suffers no important damage (other than point #1) during this time. $\endgroup$ – Loren Pechtel Mar 1 '18 at 15:34
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    $\begingroup$ @Samuel I can see how one could read "resist rapid compression" and think "more resistant than other liquids". I'll try to be more precise. Per Wikipedia "If a person were to punch or hit oobleck, it would thicken and act like a solid." I'm presuming that a bullet would be similar to a "punch or hit" by rapidly imparting force. Water also resists compression, but its viscosity does not change as shear rate changes. I.e., water has the same resistance to a bullet as it does to a butterfly landing on its surface. Not so with oobleck. $\endgroup$ – Tim Mar 1 '18 at 15:56
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    $\begingroup$ Or maybe they'll seek us to use our hides for useful purposes $\endgroup$ – Carl Witthoft Mar 2 '18 at 18:20
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Kevlar skin! Cool!

So, point of order here-- you specifically said bullet proof, but bulletproof is not knife proof. It's not at all the same kind of durability.

The most heavy duty kevlar for bullets works by spreading the impact stress in combination with ceramic plates.

So it's not that the skin has to be strong exactly, but that it can spread that stress and might have impact bones over vital areas. Bottom line is that you need layers to get what you want, and depending on the type of bullet (because there are many out there that are designed to shred through anything).

Knives are sharper and pointier than bullets. A good knife can pierce and cut through the weaved kevlar fibers made for bullets. Bullets crash into the vest with a quick, powerful shove, spreading their energy throughout the vest and stretching the fibers rather than breaking them.

So you'd want not hardness like a rock (even if it looks rocky), but some FLEXIBILITY to distribute the energy.

Weaved and layered kevlar fabric is basically like some kind of net: it has high tensile strength and can stretch and "catch" a bullet. Too many bullets or the wrong kind (pointy armor piercing) and yes, it will break, doesn't do well against slashing or piercing weapons comparatively.

If you want them stab resistant as well, that's more about layering to catch the knife.

It’s true that both types of body armor consist of strong materials like Kevlar, but it’s the way those materials are used that matters. With a ballistics vest, energy is redirected across the armor. A stab vest is less concerned about redirecting energy, and instead allows the edged weapon to penetrate into the material (that’s a critical detail). That’s where the stab vest nestles the weapon in strong materials that the edge or point can’t completely cut through.

Allowing for a little bit of penetration goes a long way when you’re talking about stopping a bullet. Sure, a stab vest might stop some of the lighter calibers on a good day. But the intermediate and larger calibers? Forget it.

Bottom line: Stab vests aren’t designed for the kind of energy dispersal that’s key to stopping a bullet.

And Vice Versa The opposite is true when it’s a ballistics vest up against a knife. That type of armor isn’t designed to trap an edge or point in its fibers. Yeah, it offers a degree of protection that could prevent injury, but don’t bank on it. SOURCE

To stop a knife you want to trap and grab. To stop a bullet you want to redistribute energy.

In both cases, layered material is key. If you design the skin specifically for bullet proofness, it's actually going to have to have immense bounce and the ability to redistribute energy without breaking. Rock-like skin isn't necessarily going to cut it, but a system of skin with kevlar-like properties, combined with plating UNDER the skin to protect vital parts could be the way to go here.

No doubt you are hoping for ricochet as a cool effect, so if you wanted to, you could put some outer plates on your creature but I would not put them on the inside of their hands.

So what do we have on the planet that is bullet proof?

There are many legends of alligators being bullet proof. They aren't but it takes more than a .22 to penetrate their hide. Take a look at this question that's right here on stack exchange. In it, there are a bunch of biologic answers to how that would evolve and what on the planet is close to that.

What you'd want is polymers probably, and a layering system like one user described as in abalone shells.

Your rock people have to have in their skin a) flexibility

b) a series of layered protection

and possibly c) curved or sloped armor.

Firstly, a projectile hitting a plate at an angle other than 90° has to move through a greater thickness of armour, compared to hitting the same plate at a right-angle. In the latter case only the plate thickness (the normal to the surface of the armour) has to be pierced; increasing the armour slope improves, for a given plate thickness, the armour's level of protection at the point of impact by increasing the thickness measured in the horizontal plane, the angle of attack of the projectile. The protection of an area, instead of just a single point, is indicated by the average horizontal thickness, which is identical to the area density (in this case relative to the horizontal): the relative armour mass used to protect that area. SOURCE

Angles are one more defense. Put that altogether, and what are the side effects?

  • Most animals with armor-like skin are invertebrates, and even those that are not are super-heavy (armadillos aren't but their shells aren't close to what you want). So no spine might be a side effect, with more of an exoskeleton deal.
  • So your guys would have to be really strong to carry around that armor, and they would likely have to consume more food than us, pound for pound and/or a very efficient digestive system.
  • Heat loss and gain. Shells can absorb heat to be used and keep it, but doesn't necessarily mean they are cold-blooded. There's at one sea turtle that is, in fact warm blooded, but their system of keeping warm is worth looking at because it is much different than our own:

The closely bundled arrangement of veins and arteries at the base of the legs, the researchers found, have a counter-current function that's the opposite as that of aquatic mammals and birds exposed to similarly cold conditions. Rather than transfer heat from (outgoing) arterial blood to (incoming) venous blood in order to maintain elevated core body temperatures while the limbs are kept cool, leatherback heat exchangers maintain higher temperatures within their limb muscles.

Their body core temperatures are typically lower than that of their muscles, and their endless amount of exercise – they’re always swimming – transfers some heat to the insulated core. This system keeps sea turtle muscles warm enough to work effectively in the cold. Leatherbacks are the sole living species of the family Dermochelyidae, which has a 50-million-year history of foraging in cool water.

Keeping heat in the muscles (and outside of the core) is especially important for nesting females, who use their legs for locomotion as well as nest digging; otherwise, they’d overheat.

If you go with that model, then your rock dudes and dudettes might be in near constant motion, using muscles as a heat sink and transference system, with a regulated core under all those layers. Overheating and overcooling might still be an issue--I don't think that they will actually be good with the extremes of either. But it's an interesting biologic twist that might be good to use.

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  • $\begingroup$ Kevlar for bullets works by spreading the impact stress in combination with ceramic plates. Not always. That's how the heaviest body armor types work, but it's more typical for a bulletproof vest to be backed with a thin metal shield, or even nothing. Soldiers have to deal with rifle bullets (and even armor piercing rifle bullets) that require a durable back stop, but for most law enforcement and civilian uses, where you're going to be dealing with the much slower bullets fired from handguns, Kevlar alone is enough to stop those bullets. $\endgroup$ – HopelessN00b Mar 1 '18 at 22:19
  • $\begingroup$ @HopelessN00b This is true. : ) Depends on the caliber and type of bullet. Figured the poster wanted the full treatment. $\endgroup$ – Erin Thursby Mar 1 '18 at 23:42
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Side effects:

  • some medical procedures, not only surgeries, but even simple injections would become quite tricky (actually in modern society it may negatively influence life expectancy)

  • this race would presumably not use tatoos...

  • their pelts would be quite valuable on black market

  • if they share environment with humans, they would presumably end up as a warrior caste (or just overlords)

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    $\begingroup$ No tattoos but possibly etched/spray painted graffiti. $\endgroup$ – cup Mar 1 '18 at 14:04
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They might well require stronger skeletons and bigger muscles than your average human in order to carry their natural 'armour'. While lighter materials do exist that meet your bulletproof criteria it seems unlikely that evolution would produce an organism able to produce them in sufficient quantities to cover a humanoid creature.

Their sensory organs (assuming that these are squishy and not hardened) would need to be further from the internal components of the body. Therefore they would have longer nerve connections to them which might increase the risks of damage or sensory disruption.

As already indicated in the comments they would not necessarily be hairless. Plenty of animals with tough skin have hair - e.g. elephants or rhinos. However, based on these examples it is plausible to think that their hair might be quite sparse.

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    $\begingroup$ Elephants and rhinos don't have sparse hair because of their thick skin - it's because they are big, and bigger animals have a hard time staying cool. Note that the honey badger, which also has very thick skin, is quite furry. $\endgroup$ – IndigoFenix Mar 1 '18 at 15:39
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  1. If their skin is grey, it probably doesn't have melanin, meaning they might be more strongly affected by ultraviolet rays and more at risk for skin cancer, unless their skin has a substance which fulfils the same roles as melanin, just grey in colour. They also wouldn't get tan lines. (Probably. They might just go from darker to paler from sun exposure, and not the other way round.)

  2. They might be unable to produce vitamin D.

  3. Ingrown hairs are a pain to deal with.

  4. They couldn't get tattoos or at least not the kind with a tattoo gun. I guess it depends on how resistant their skins are for those. Same with piercings. You'd need to decide whether they would develop different methods for tattooing and piercing, or just not bother.

  5. Do they have fingerprints? And does their skin produce oils?

  6. Would they get wrinkles as they age? It seems to me that wrinkled skin might not be that good at repelling gunshots.

  7. Would their skin bruise? The discolouration might not be visible under the grey layer. They might be unable to blush or pale, so they may appear more stoic and emotionally less accessible to other people.

  8. Would their facial muscles be able to express emotions to the same degree as humans?

  9. Are their skins also the texture of rock? If yes then their clothes may need to be designed from more resistant materials. Also furniture and whatever objects they need to touch constantly. Think of touch screens being scratched up by rocky skins. On the bright side, they might be highly sought after in the pedicure industry!

  10. The fashion and cosmetics industry would probably develop products specifically to fit their needs/skin tone.

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  • $\begingroup$ Would it be possible to have surgery? Not being able to is a pretty big minus. $\endgroup$ – Clearer Mar 1 '18 at 10:31
  • $\begingroup$ @Clearer I'm not a doctor, but I think surgeries are generally not performed with projectile weapons. I could be wrong, though, I don't know any particulars of this universe. $\endgroup$ – Real Subtle Mar 1 '18 at 12:56
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    $\begingroup$ @Clearer There's a difference between bullet proof and knife proof, and things built to be bullet proof are not generally knife proof and vice versa. $\endgroup$ – Erin Thursby Mar 1 '18 at 13:24
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    $\begingroup$ @Clearer That's a tough build to do! We have trouble even manufacturing things that can do both well, so from a biologic standpoint that would be super crazy! Interesting though, like you say. $\endgroup$ – Erin Thursby Mar 1 '18 at 14:39
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    $\begingroup$ "Impossible" it is a bit strong word, nevertheless making an incision with a scalpel in skin that is as resistant as bulletproof vest may be a bit time consuming. This race should be able to develop ruinous medical bills much faster than humans. $\endgroup$ – Shadow1024 Mar 1 '18 at 15:06
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All the other answers are good, but they've missed this:

Surgery

If the Rockfolk suffer from internal bleeding, they die. If your Rockfolk have cancers, they die. If your Rockfolk have any internal problems, they die.

Pregnancy

If your Rockfolk need a C-section, they die. Note: It would be pretty cool if their unborn children can survive inside their dead mothers. If so, you could have some spooky unborn children wreaking havoc in your world :)

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  • $\begingroup$ Welcome to the site. While you make interesting points and ask interesting questions these are commentary on the answers not an answer to the question that was asked. Check out the tour and help center to get more info on the site. Happy world building. $\endgroup$ – James Mar 1 '18 at 21:41
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I'd make it so that their skin is much more rigid, so it is a problem when they grow and they need to shed once a year like a snake does.

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  • $\begingroup$ Welcome to the site. This is more commentary not an answer to the question that was asked. Once you earn more reputation on the site you will be able to comment. Check out the tour and help center to get more info on the site. Happy world building $\endgroup$ – James Mar 2 '18 at 14:57
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You need to define "tough" first.

Obviously, "tough" does not mean rigid, since you said the skin was flexible. If that is the case, then the underlying tissue would still take the impact force, even if the skin does not break. "Tough" also cannot mean inelastic, because otherwise movement would be very difficult. Again, the tough-but-elastic skin would allow the impact forces to be felt by the underlying tissue. Basically, I don't see how you can protect the underlying tissue unless the skin is rigid -- which you said it is not.

So, I am suggesting that you cannot have what you want: flexible yet rigid enough to be bullet proof. If you want flexibility, then the skin may be bullet proof, but the tissue underneath will suffer nonetheless. And that't not really bulletproof, is it?

Some people have suggested non-Newtonian, dynamic substances that resist rapid changes in shape. Great idea! This has actually been used in bulletproof vests, filled with shear-resistance fluid chambers or something similar. But this would make the skin bulletproof, but not slow-knife-proof. Is that what you want? It's still a pretty good trick, but not very rock-like for your Rockfolk.

As others have said (a la Nick Cage) if you're skin is too tough, you cannot undergo surgery or be injected with hypodermic needles -- or any number of desirable actions that involve piercing the skin followed by healing.

Does your bullet proof skin grow as the Rockfolk mature? If so, then that introduces the possibility that the skin can be breached: The adjacent molecules that make up the skin, or the cells that are its building blocks, must be able to separate to allow for new material to be added. How does that occur? Can a chemical mean me used to hijack this mechanism, to weaken the skin?

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  • $\begingroup$ Welcome to the site. This is more commentary not an answer to the question that was asked. Once you earn more reputation on the site you will be able to comment. Check out the tour and help center to get more info on the site. Happy world building $\endgroup$ – James Mar 2 '18 at 14:57

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