You're looking too much in to it. We're talking manga physics here.
Where rubber is stretchy and diamond is hard. If you want to take this so serious maybe we should ask ourselves why Luffy has never snapped like a rubber band. Because that is the fate of rubber.
If Oda depict Jozu as a diamond man, he is saying this is a tough guy to hurt, this is one of the best defenses in the world. Brook can heal his bone injuries pretty fast by drinking a lot of milk. That is another hyperbole of a real world appliance used in the One Piece world.
We can agree to disagree but I don't think that Oda would make Jozu brittle to blunt force.
I'll have to respectfully disagree, my friend. I'd say you're not looking into it
enough.
While One Piece clearly isn't THAT scientifically accurate it isn't completely without science either. Yeah, it's shounen, but that doesn't obligate or insinuate that it stick to 1st grade fundamentals. Look at Bounce Man, for example. It is literally the application of CoA to manipulate one's own elasticity to the point of turning their arm into a freaking canon, dude. One Piece is more than just "Rubber is stretchy, kids!", you know?
Luffy is superhumanly durable like many others in the OP-verse. That would be why his rubber body doesn't snap from all if his exports. Also, we know there IS a limit to how much he can stretch without snapping, as stated by Oda and shown in WCI before Jinbei's rescue act.
See, arguing for author portrayal is nigh-IMPOSSIBLE. Unless you've spoken to Oda or have a statement, I can't see how you'd know his position on the matter. Though, if you want to use his work as evidence I'd like to redirect you to Bounce Man. Also, Red Hawk. That shit is legitimate science. Rubber can self-ignite through the adiabatic process. If Rubber is stretched very quickly before the heat can dispel it will be hot. You can try this at home with your everyday rubber band. With enough force and a high enough velocity one can stretch a rubber band and make it burst into flames. Add Luffy's durability and haki to the mix for heat/force-resistance and more brute force and there you go- Red Hawk. Is that not science? It's also simple enough to perform without Luffy having to know anything about thermodynamics. As far as he's concerned, if he sends a haki punch super-duper fast it makes a big BOOM and that's all he really
needs to know.
What about Caesar using his Gasu-Gasu to remove all of the oxygen within a certain radius, thus turning it into a space filled with nothing but nitrogen, a deadly poison if consumed on its own?
In light of these many post-elementary applications of physics and general science I'd say something as simple as giving Jozu's diamond regular (but highly exaggerated) properties of diamond or at least the same basic composition isn't at all a stretch due to Oda not expanding on science. It's something he actually does pretty often.
Though, we can disagree.
Sorry for Bible #2. Don't ban.