You can smash a diamond with a hammer or a hydraulic press. Hardness isn't toughness. Hardness is a measure of structural resistance to deformation. ("Squishing")
Take a piece of steel for example. Steel is wonderful because it can be hardened to a relately wide range of values.
Mild steel is relatively soft. If you hit it very hard, it will bend, and stay bent. If you harden it, however, it won't bend when struck, but will instead break. With the right amount of hardened steel and softer steel, you get a spring, which bends, but returns to its previous shape. Of course, you can still snap spring steel, it just takes a lot more work.
To answer the question above, a diamond blade would be sharp as fuck, but shatter if you hit anything substantial with it. The best way to use diamonds in sword design is the same way they do in industrial applications: use the diamond only for the edge.
There are a number of creatures that are immune to bludgeoning damage from non magical attacks, but the demogorgon seems to be alone in having immunity to non-magical bludgeoning damage regardless of the damage coming from an attack or any other source.
I think it would probably be slashing immunity and perhaps vulnerability to bludgeoning if you were to represent the properties of diamonds in mechanical D&D terms; they're very hard and pretty scratch proof, but will sheer or shatter far more easily
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u/paladinLight Blood Hunter Nov 13 '21 edited Nov 13 '21
Diamonds are hard, but torsion force will shatter them, and they can be pierced by a drill.
Diamonds just got Bludgeoning Immunity.
Edit: they actually have slashing immunity, and possibly bludgeoning vulnerability.