A medieval alchemist (from Europe) has figured out how to crystallize any object and turn it into diamond. Would diamond armor (e.g. chain mail and plate armor) and weapons be superior to their traditional counterparts? They would be lighter (density of diamond is 3.5gm/cm^3, roughly half of the density of steel/iron). However, I am concerned that since diamond armor would be so inflexible, it wouldn't absorb much of the energy of a hit.

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    $\begingroup$ Diamonds are brittle, so it wouldn't be good idea - it may absorb energy, by breaking into pieces. $\endgroup$ – Mithoron Apr 22 '16 at 16:05
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    $\begingroup$ @SJuan76 The alchemist can crystallize any object and turn it into a diamond, so he can take a finished piece or armor and turn it into diamond. $\endgroup$ – Ovi Apr 22 '16 at 16:39
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    $\begingroup$ At first I thought this question was going to be about MineCraft... $\endgroup$ – Michael Apr 22 '16 at 19:13
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    $\begingroup$ Diamond weapons might work out better than armor. $\endgroup$ – Molag Bal Apr 22 '16 at 23:18
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    $\begingroup$ One thing that doesn't seem to have been menioned yet is that diamond is transparent. So in a medieval society in which glass can't be made strong or in large amounts, diamond visors would probably be quite useful. $\endgroup$ – Nathaniel Apr 23 '16 at 0:15

The material property we'll want to concern ourselves here is Toughness. This value denotes a material's resistance to fracturing.

A typical diamond has a toughness value of 2.0 KIc, which is greater than most stones or rocks, and similar to ceramics. Steel, on the other hand, has a value of 50 KIc, and many metals have similarly high values compared to diamond.

In short, armor made from Diamond would likely shatter at first contact with a weapon, and would not be suited to use as a traditional armor of any kind.

That being said, there is some possibility that an armor that fractures upon receiving a blow might be useful against certain projectile weapons, since the impact would be distributed among the fractured pieces, but this protection would be a one-off, and there are ceramics more suited to such a purpose than diamond.

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    $\begingroup$ There totally needs to be a Minecraft mod that turns diamond gear into such one-off super-protection. $\endgroup$ – John Dvorak Apr 24 '16 at 6:28

Diamond is a material with a very high hardness, but is not supple or flexible like metals.

By analogy, look at a traditional Katana. The blade is actually a complex three-dimensional structure, with levels of harness being created by controlling the tempering of the metal (for example, coating only part of the blade with clay when heating and quenching, so the coated parts have a different temper and are softer than the exposed cutting edge. (The hamon; wavy pattern along the side of a traditional Katana marks where the clay coating was applied.

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A diamond edged blade made along the same principles would be incredibly sharp yet still be flexible enough to use without worrying too much about breakage. For movie fans, you could create something like the "Green Destiny" sword from "Crouching Tiger Hidden Dragon".

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As for armour, pure diamond will suffer many of the same issues as using diamond as a blade. A diamond coating over steel plate or steel rings will make the armour stronger and more capable of resisting a strike by a weapon (diamond edged or not), but there could be issues of the diamond coating coming off the steel under layer due to the force of the blow. Possibly the best way to reduce this is to make your armour in scale form to allow for flexibility and absorb blows without separation

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    $\begingroup$ Carrying the katana principle to an extreme, there's the Aztec macuahuitl: a wooden blade set with brittle but sharp pieces of obsidian. $\endgroup$ – jamesqf Apr 22 '16 at 17:20
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    $\begingroup$ The only reason the katana construction works is that the different phases of steel have the same Young modulus. If you tried it with diamond and iron, you may as well have made it hollow; a good impact will make the diamond layer shatter and leave you with a useless iron core. $\endgroup$ – Mike L. Apr 22 '16 at 20:30

Diamond armor would have to be constructed very differently than traditional mail or plate armor for it to work in the manner intended.

As @MozerShmozer pointed out, Diamond has a very small toughness, and viability to cracking, so diamond plate armor is out. Like you stated in the OP, even a single layer of diamond mail would slowly disintegrate over multiple hits, even if scaled like @Thucydides suggested.

However, if you created armor with multiple layers of diamond scale, you would have something that is about the same weight as normal armor, with added protection from projectiles due to the diamond scales shattering away . The only issue that would arise is if a foe were to forcefully impact the exact same spot multiple times, which typically doesn't happen in a normal battle, until all the diamond scales in that one spot are gone. Depending on the thickness, a warrior with diamond scale armor could conceivably wear 3 or more layers (or even more layers on specific body locations) that would have the same weight of a typical armor of chain or plate mail.

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    $\begingroup$ That is quite clever! It would obviously be prohibitively expensive irl, but in OP's scenario diamond could conceivably be quite inexpensive if the alchemical process is simple enough. This could lead to very interesting tactical considerations as well, where armored warriors move about as the battle continues and have to change their fighting style to protect areas of his armor that have been weakened, and exposing fresh armor to absorb future blows. $\endgroup$ – MozerShmozer Apr 22 '16 at 19:12
  • $\begingroup$ I didn't think of that, but you're right. Also, as the battle drags on, those layers of diamond scale will slowly get lighter over time, theoretically allowing fatigue to set in slower than normal fighters with constant-weighted armor. $\endgroup$ – Vogie Apr 22 '16 at 19:49
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    $\begingroup$ An armour that needs replacement after every hit is not terribly useful. Brigandine faced with something like leather might be a better idea; it would keep the diamond scales from shattering and they in turn would stop penetration, with some padding underneath dissipating the impact further. $\endgroup$ – Mike L. Apr 22 '16 at 20:38
  • $\begingroup$ This seems very similar to the idea involved in bulletproof glass $\endgroup$ – March Ho Apr 23 '16 at 0:09
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    $\begingroup$ @MikeL. Many people in battles never get hit and those that do probably don't get hit many times (or they'd get hit somewhere armor does not protect and be incapacitated), especially in the same spot. Modern troops use ceramic plates that similarly need replacing after a hit. $\endgroup$ – Hassassin Apr 24 '16 at 6:46

Diamond may not be hugely useful as armor, but It would be fantastic for arrowheads, Javelin points, lance points or other "discard able" weapons.

the point (hah) of these weapons is that they only have to puncture a foe once, not to be used again and again. They should be good at holding an edge and puncturing things like leather and chain mail armor. This would be similar to flint and obsidian arrowheads that were fragile, yet incredibly sharp.

The possibility of shattering is actually attractive, as little tiny sharp bits are hard to find and extract bring the possibility of infection and sepsis.

Finally, can you imagine the shrapnel possibilities in a diamond medieval hand grenade? Yikes.

  • $\begingroup$ I don’t think that’s right. It will shatter and not penetrate. Small shards left from it will be even less effective. $\endgroup$ – JDługosz Dec 28 '16 at 2:50
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    $\begingroup$ Possible, I don't know beans about diamonds in particular. I do know that flint was a very useful spear and arrow point material for millenia. Likewise with obsidian. Both of those materials are very brittle, which is why I would use them for single use items like arrows and spears. Diamond should have very little problem with leather and chain mail armor, as the chance of a direct impact, point on steel, is unlikely. I'm less sure about efficacy against plate armor. Also, small shards of glass get everywhere, and even small cuts on a battlefield are an invitation to sepsis. $\endgroup$ – Paul TIKI Dec 28 '16 at 18:49
  • $\begingroup$ I see… you should make the comparison with flintpart of your Answer. Maybe even look up the numbers explained by MozerShmozer. $\endgroup$ – JDługosz Dec 28 '16 at 19:29
  • $\begingroup$ Flint was useful material for spears and arrowheads that were very sharp and good for cutting soft(ish) targets made of meat, leather and fur. A diamond arrowhead hitting the middle of a reasonable 10mm chainmail ring would (assuming that the head is wider than 10 mm, not a "needle") mean shattering after penetrating just 10-20mm beyond the mail - comparable to the quilt padding thickness, so the shards would barely reach the skin at most. Compared to that, standard bodkin arrowheads were far more effective at piercing mail, so making them of diamond would just make them less useful. $\endgroup$ – Peteris Dec 29 '16 at 1:25
  • $\begingroup$ Interesting. I would think that an arrowhead would penetrate deeper through chainmail, but with what you just pointed out, the shape of the head would be critical. Now I'm trying to think of a way to use diamond as a shrapnel agent with an arrow as a carrier. Oh well, it sounded really cool in my head. I guess it's all dependent on he level of tech in this world. It would probably work in a Bronze age civilization, but after the iron age, it stops being useful $\endgroup$ – Paul TIKI Dec 29 '16 at 14:42

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