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
b) a series of layered protection
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.