How feasible is it to make a blade out of pure diamond?

It would be sharp but very brittle. It would only be good for the first strike.

Yes, very brittle.

http://en.wikipedia.org/wiki/Synthetic_diamond#Chemical_vapor_deposition

Looks like it could be done, theoretically.

I wouldn’t want it to be my main weapon, however. Good ceremonial sacrifice weapon, bad fencing sword.

best question ever? possibly.

They already use diamonds for surgical instruments but these things are very small and are treated gently. It’s not robust enough for anything bigger.

Yeah, it would not do well as a combat blade. The surgical instruments they make with diamond are excellent- good at cutting through things like bone. I believe they use them in some diamond-tip drills as well- those can drill through pretty much anything. Not for a larger blade though- making a diamond that big would be a hassle too.