Let’s say we had the technology to make single-crystal artificial diamonds in any size and shape we wanted. If someone were to make a knife or sword out of solid diamond, would it actually be practical as a weapon? I’d imagine it would hold an edge very well, but would it shatter the first time it hit something hard like metal or ceramic?
There’s a science fiction book called The Diamond Age by Neal Stephenson wherein people build things out of giant, artificial diamonds.
I have no idea about the answer to your question, though. 
I thought Diamonds were the hardest thing on earth, next to Ron Jeremy. Why would they shatter if you hit something with it?
You hit it right on the money- a diamond sword would shatter way to easily to be of any practical use.
Diamonds are hard, very hard, but they are NOT shatter proof…
How do you think they are able to be made in to all those fancy shapes?
While they are extremely tough, a diamond can be fairly easily chipped and shattered if hit at the proper angle.
If they could control the way the diamond is formed on a molecular level, I would think that it would be very strong, (nearly) unbreakable and as flexible as they would want it.
Or they could make it out of BuckyTubes (now called nano-tubes?) it would have whatever properties they would want then.
One thing though, if they had the tech to make that why would they? They would make LightSabers instead 
Diamonds do “cleave” if you hit them right. Learning how to do it right is trickt and expensive. I know, because I looked into it once. I never got to cut diamonds, but I’ve cleaved (cloven?) a lot of other crystals. If your sword was a single diamond, I’ll bet that sooner or later you’ll hit something just right and you’ll break it.
Hard does not equal strong. In fact, if something is very inflexible it tends to be very brittle, and vice-versa. That’s why sword makers have sought so long for solutions that give you a sharp, keen edge without having the sword break. Two different solution emerged – the somewhat flexible blade with a tempered edge (as with samurai swords, where you can see the transition zone at the edge of the tempering), and slotted swords, as made in Renaissance Italy.
There are differences between strength, hardness and, most important for weapons, resiliency. Diamonds don’t bounce back.
With that level of technology, they could make steel swords that were perfect at the molecular level. Now that would be a kickass sword.
Slan, anybody?
Depending on the type of sword, too, you do want it to bend and give at times.
I thought about a lightsaber, but couldn’t figure out any way to make one with any plausible technology.
Sounds like a sword of very good steel, possibly with a microscopically thin diamond coating on the edge, would be the best bet.
That sounds really kool! What about titanium? Can titanium keep a good edge? How about a titanium-steel alloy? Or that plas-steel stuff?
Ah screw it just go with Adamantium…or Mithril.
Now that I think about it, what about a vibro-sword- A sword where the blade vibrates at 10000 htz or so? (the hilt would have to be insulated against this). Or a sword that heats up to a very high temperture? Or even a sword with a molecular sized chainsaw chain around it? Or for that matter, what about a sword that is only a few molecules thick? Still though, I don’t think people that far advanced would use swords for anything other than sports.
BioHazard, what are swords used for nowadays anyways? Mostly just for exhibitions and sport…
…and 80’s metal videos of the Iron Maiden/Dio ilk. (side note: for the highest of high comedy, try to watch Dio’s “Holy Diver” video, just trust me)
As per sword-making material, I’d say Adamant (used by the Dwarf sword-maker in the original Final Fantasy for 8-bit NES) is tops.
BioHazard, what are swords used for nowadays anyways? Mostly just for exhibitions and sport…
…and 80’s metal videos of the Iron Maiden/Dio ilk. (side note: for the highest of high comedy, try to watch Dio’s “Holy Diver” video, just trust me)
As per sword-making material, I’d say Adamant (used by the Dwarf sword-maker in the original Final Fantasy for 8-bit NES) is tops.
:dubious
Home defense! I have my well-used Scottish claymore hanging over my fireplace and in the confined space of our house, I would defy anyone to get their gun pointed at me faster than I could crush their clavicle.
I’ve got other swords and axes and knives strewn about the house as well, hmm… maybe I should get a life? naaaaaa…
Now, about the OP. any sword-blade sharpened to the nth degree is no longer a sword, in my opinion. It’s a cutting tool akin to a lazer beam. Swords need weight and resistance or you would not be able to control them. A real lightsabre user would cut his dang-fool head off inside of 2 minutes during combat, unless the force also existed.
Wait a minute… what was the question again?
Oh, a soft steel blade with a molecular-diamond edge would be fun.
I would think a diamond sword would be rather strong. If I remeber correctly, on the scale they use to measure hardness of rocks, diamond is the hardest, followed by corundum. (Wow, I actually did learn something in Earth science.) This is why they are used on the tips of the blades that dig into the earth.
When I moved into my new apartment I made sure that the locals saw me carrying my two handed barbarian sword and battle axe to forestall any thoughts of thuggery.
This has been discussed, and unfortunately (it would look cool) diamond swords are probably a no. On a scale of 1-10, Diamond was 10, and (AFAIK) Talcum was 1. That’s only hardness, not give, remember.
Later in the same book, a character makes a short sword with a micro, diamond tipped chainsaw edge that spins at some huge rpm value, like >20k rpm.
Gives new meaning to the phrase… “let the knife do the cutting”
AndrewL / BMallion
diamond-edged swords
Now, that would work: same principle as Aztec swords (wooden base material with embedded obsidian blades).