Lets get some things straight...
Diamonds are the hardest substance known to man, yes? Just Google it. BUT the hardness rated on the Mohs scale is based on “… a qualitative ordinal scale characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material.” according to Wikipedia. So this Hardness is relative to Scratching the surface.
Diamonds though are also very brittle. You can easily smash one with a hammer. They can also be burned. Here is a very informative video on the subject. Here. In the video he used an uncut crystalline diamond. the only difference between that and a cut an polished diamond is one has been faceted. It is still nothing but carbon. Why do I bring all this up? Sooooo many Steven Universe “who shattered Pink Diamond” theories are operating under the idea that “Only a Diamond can Destroy a Diamond”
this is False.
A hard enough strike will break any Gem, hence Bismuth’s Breaking Point.
That would do it. A sword though? probably not. Not enough heft to it unless it was a pommel strike.
Edit: I’m not saying it isn’t possible it was one of the diamonds, just that it isn’t only a diamond that can destroy another diamond.
tl:dr Diamonds can be smashed like glass.