Okay I've heard this about 8 billion times at this point and it seems to be a focal point in many theories, and I'm gonna smash it into little itty bitty pieces for you.
Okay, see those things?
Yes, those. THEY ARE NOT DIAMONDS.
NOT. DIAMONDS.
I cannot even begin to explain how this is the utter bain of my existance in this fandom, these are absolutely not diamonds.
Proof?
*Cracks knuckles*
See that in the original drawing? Replacing Rose? That is Pink Diamond. There is clearly a Diamond on her chest, and on those other things? Those are triangles. Not. Diamonds.
Don't believe in the mystical being that is PD? All good, there's still a Diamond in the original mural, held by Rose Quartz. The shape still doesn't match the ones on the little triangle dudes.
"So what are those things?!" You scream angrily into the void that is the internet.
Well boy howdy, do I have answers for you lil buddy. They're the Diamond Authority Military.
My proof? One of them is fighting Rose in the final product for one. Do you honestly think a Diamond would sink down to the level of fighting physically, and risk their own gem rather than just commanding another gem to risk themselves? I doubt it.
Secondly, Blue Diamond was the only Diamond on earth during the rebellion, aside from perhaps Pink Diamond.
Thirdly, this mural depicts the last bit of the rebellion as was also depicted by Lapis. Rose Quartz wins a battle against homeworlds finest military whilst keeping them from coming near the kindergartens,
Homeworld says "balls to this" and hightails it out of earth asap while leaving all "unnecessary" gems who saw their defeat behind,
and then finally, to make sure that the mess cleans itself up, Homeworld detonates an EMP bomb to damage all gems on earth to the point of becoming corrupt. All except those Rose Quartz had inside her shield.
So now, I hope you all see that those triangle guys in the mural are most definitely NOT Diamonds, and that this clears up all the confusion.