I don't know how SU fans manage to put so many spins on one theory, but here is a very very new one with some brand spanking new evidence. It may just surprise you.
Bismuth said that Rose was “Just another Quartz made right here in the dirt”, which SHOULD completely disprove the Rose Quartz is Pink Diamond theory…
Yet it ended up being the final clue to prove that she in fact is.
Bismuth said “here in the dirt”, meaning she believes Rose was made on Earth. This, however, is entirely untrue. In “On the Run”, it is heavily implied that Rose originates from Homeworld and in “Rose’s Scabbard”, Rose says “if we win, we can never go home”, meaning her and Pearl have the same home, or place of origin, and we know as a fact that Pearl comes from Homeworld. Pearl has known her longest, knew her best, and thus would definitely know Rose’s true place of origin.
Meaning Rose flat out lied to Bismuth.
Rose is a quartz, and yet she was made on Homeworld. In “When it Rains”, Peridot says to Steven, “You’re some kind of quartz right? You must have been made here.” Peridot, a gem created to obtain information, is certain Steven was made on Earth. She doesn’t ask him which of these several planets could you have come from, but instead confidently states that he was made right there. This, if anything, means that Quartz are likely found explicitly on Earth.
And this is what Bismuth, and likely the rest of the Crystal Gems, were made to believe. This not only raises the question as to why Rose would lie in the first place but to how it was even possible for a Quartz to be formed on Homeworld.
I believe part of the answer lies in one of Steven’s more recent power developments, that to take control of another’s body through his dreams.
We’ve only seen him take over Lars, but the manner in which he entered Lars’s mind is exactly the same to how he entered Kiki’s, and more importantly, Lapis Lazuli’s when she was in Malachite.
If Steven had just been harmlessly passing through, and not fighting for control over Lapis’s mind, Lazuli wouldn’t have struggled as much in his presence. But in fact Steven was subconciously trying to take control from her, and Lazuli was forced to fight against not only Jasper and Malichite, but Steven as well.
Undoubtedly Rose had the same power, but as she had a much stronger control over her abilites was likely able to fully control gems.
Rose’s whole gimmick is defense, and this ability would have served as the ultimate form of self-protection. Shecould have taken the body of any gem, could go into battle without risking her life, could command her army through another’s voice while safely hidden away somewhere Homeworld couldn’t get to her.
But this isn’t Rose’s style. If she were to fight with another gem’s body, that would be the same as saying that gem’s life was less important than hers, which is exactly what Rose and the rebellion were concretely against. They were fighting for equality for all gems. This is the same case with Steven, as he would never want to put anybody in danger at his expense.
So why would the Crewniverse even bother giving Steven and Rose a power like this, if neither of them would ever end up using it? Even in a dire situation, it completely goes against the nature of Rose and Steven. It just doesn’t make sense.
Now there may not seem like there’s a connection to all of this, but I’d like to resolve that by having you think of this scenario:
Imagine you’re a common gem with a set purpose, who’s fed up with the diamonds and aristocrats telling you what you have to be, and have become a rebel to fight such injustice. How likely is it you’d want to charge into battle under another Diamonds command? A Diamond, the very thing you are fighting against, the representation of everything you believe is wrong with society.
And who’s to say that this all powerful gem didn’t just get bored, and decide to wage a war for their own entertainment? It would be impossible to trust let alone follow such a leader. But what if instead of a Diamond, an aristocrat, it was a common gem, born on the very planet you’re defending, that decided to start the revolution.
Not only would you be more inclined to trust one of your own, but it would be much more inspirational seeing an average gem stand up to the Diamonds rather than an immensely powerful one.
Here’s what I’m getting at: what if Pink Diamond, Rose Diamond, body jumped into an earth-grown quartz to protect the Earth from the Great Diamond Authority.
This is why Rose has all of those incredible powers, despite supposedly being just another quartz soldier. She disguised herself in another body, likely turning a regular clear quartz into a Rose Quartz, to inspire gems, telling them that she was made on Earth like many of them were.
This would also explain why none of the statues of the Diamonds look exactly like her, or why she is significantly shorter than the rest of the diamonds.
We know from the insignia in the sky arena that Pink Diamond was present prior to the revolution, but as seen from Centipeetle’s drawing in “Monster Reunion” by the end of the war Pink Diamond had left the Authority.
In Centipeetle’s drawing of how she was corrupted, Yellow Diamond appeared to be the only one present during the corruption of all the gems on Earth, but when Steven asked who was responsible Centipeetle drew all three diamonds. This is because while White and Blue Diamond may not have been directly responsible for the final attack, they fought against Earth to the end, and were thus equally liable for the outcome. Following this logic, that means that Pink Diamond never could have been involved in the revolution, or at the very least she didn’t side with Homeworld.
Rose’s body jump would have been her most guarded secret, something that not even the Diamonds would have known about. If Homeworld had known what she’d done and told the Crystal Gems that their leader had lied and ultimately betrayed them, the revolution would have completely fallen apart.
Rose would have also wanted to keep this secret to protect her court. If Homeworld knew Pink Diamond had betrayed them, they would have mercilessly shattered her entire court as either traitors or potential traitors. Rose likely quietly dissappeared, leading Homeworld to believe she had vanished, perhaps in all the commotion of the rising revolution.
As to the body Rose jumped to, we know Rose wouldn’t have wanted to take the body or identity of another gem for an elongated period of time and I have a few theories on this. One is that the gem was somehow defective and hadn’t been able to properly form sentience. Another, more likely idea is that Rose jumped to a gem that had not yet formed, but was still in the ground, yet to create an identity for itself. This would have been incredibly difficult, but for Rose perhaps not impossible.
Then what would she have done with her original, diamond body once she’d jumped into Quartz’s? She wouldn’t have wanted to just stash it somewhere some gem could have discovered it. Not only could it possibly be used for ill intented purposes, but it could potentially reveal the fact that she was Rose Diamond. The location of her body would undoubtedly be one of her best kept secrets.
Pearl, her second in command and longest standing ally thought she knew everything about Rose, which was shown to be untrue.
And her one best kept secret, one even Pearl didn’t know about, was Lion.
We know gems can shapeshift. We also know that they can create sentient life in objects outside of themselves, such as Lazuli’s water doubles or, huh, Steven’s cat fingers.
So, what if Rose made her body one giant, sentient cat finger. Or, you know, a big pink lion.
I’m not saying that Rose is Lion. Just that she shifted her previous body and imbued it with its own sentient personality.
Then, she could hide her body in plain sight, while always keeping an eye on it. Not only that, but she could trust it with all of her secrets, as it was literally herself.