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.

[![enter image description here][1]][1]
[![enter image description here][2]][2]

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".

[![enter image description here][3]][3]


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

[![enter image description here][4]][4]


  [1]: https://i.sstatic.net/P3R68.jpg
  [2]: https://i.sstatic.net/Q8BQ5.jpg
  [3]: https://i.sstatic.net/zXTwZ.jpg
  [4]: https://i.sstatic.net/T2zIr.jpg