You can't cut diamond. If you put a knife on the diamond first and then apply force to the edge of the knife, the diamond will not get cut. But if you use a diamond sword and try to make a cut on a plastic bin (
Like this plastic bin) (like swing the sword to the plastic bin), the diamond will... idk... crack or break. I don't know how tough diamond is. Here's the definition of hardness and toughness of a substance. (Copied from wiki) And then I compare diamond with a metal, like a knife.
"Hardness is a measure of how resistant solid matter is to various kinds of
permanent shape change when a
compressive force is applied." You know that knife can get blunt (Bad English? Sorry) when you use it a lot and never sharpen it. So that means that knife is not really hard because you can change shape easily. But if you try to bend something like a diamond stick... wait, you can't even bend diamond, or change its shape.
"Toughness relates to the ability to
resist breakage from falls or impacts." You don't see everyday metals that can break, right? So that means the toughness of metal is, well, really tough. But diamond is not really tough. So, it can break easily.
Yeah, chemistry!