"So the whole time you were dealing with Rose, you were actually dealing with Pink Diamond."
After Steven managed to get the White Diamond to cave (long story) and admit he wasn't the Pink Diamond she remembered, he gathered all three Diamonds and the Crystal Gems into the throne room, where he told them all (he knew) about his gone-but-not-forgotten mother, the Pink Diamond.
Pearl sidled up beside him.
"Why are we doing this in the throne room again?" she asked, hesitantly.
"To make it official," said Steven.
Pearl slipped back to Amethyst and handed her a fiver.
The Yellow Diamond sighed, "It seems we owe all those bubbled Rose Quartzs an apology."
"This is really embarrassing," the Blue Diamond sobbed, "We're the victims of a hoax!"
"THIS IS AN OUTRAGE! I WAS THERE! THERE WAS A ROSE AND SHE SHATTERED HER!" yelled the Eyeball Ruby...wait, what?
Pearl slipped up to Steven again, "What's that Ruby doing here?"
"She was the only witness," Steven quietly explained, "I figured it would be easier if everyone was on the same page."
As Pearl backed away again, Steven turned to address the Diamonds once more.
"As you already know," he declared, "White was the only one who knew the truth, as she taught Pink Diamond how to shapeshift in her youth. She was the last Diamond to join the Authority, and therefore the youngest."
"I still find it highly outrageous that Pink ended up turning against us, running away to form a," the Yellow Diamond paused, shaking her head in disgust a bit, "Ragtag bunch of misfits!"
They've heard worse.
"To be honest," said the White Diamond, "Even I didn't see this coming."
"Hmmm..." then Steven had an idea, "Do they take photographs and audio recordings on Homeworld?"
"Yes, why?" said the Blue Diamond, calming down a bit.
"Maybe Pink deliberately left behind clues of her eventual alleged shattering, and you didn't notice."
"Are you saying Pink was somehow foreshadowing her leave?" said the White Diamond in surprise.
"Hmm," pondered the Yellow Diamond, "That's an accurate way of putting it. Very well. As the 'son' of Pink Diamond, 'Steven', you have unlimited access to the Archives of the Great Diamond Authority™."
The triangular platform that he stood on during the last trial lifted a bit, then a drawer opened on one side. Longer than one would expect, it was chock full of photographs and tapes, many of them predating the Pink Diamond's going AWOL: just what Steven needed.
Five minutes later...
With help from the Yellow and Blue Pearls, Steven assembled slides of the pictures he chose as evidence into a carousel projector. Off to the side was a working portable tape recorder, capable of playing the kind of tapes the Diamonds used back in the day. Amethyst turned into a laser pointer for him to use as well.
"Now, there might be more clues elsewhere in the archives, but these are the ones that I felt stood out the most. You can find the others at your own leisure," said Steven, turning on the projector, "Okay, here's the first clue."
-Clue #1: "This is the last known photograph of the Diamonds with Pink. You might already notice that Pink is barefooted, but if you look a little closer, there's a shard of a shattered gem, peeping between the fingers of Pink's right hand.
-Clue #2: "Here's a picture of Pink doing a walkabout on Earth, in what I'm assuming is Ancient Rome. There's a stone archway behind her that reads MDIF. MD is 1500 in Roman numerals, making the inscription read '1500 if'. So that means if Pink Diamond was shattered, she'd be 1500 years old.
Steven looked toward Pearl.
"Pearl, how old was Pink when she became Rose full time?" he asked.
"At least fifteen thousand, give or take a year," admitted Pearl.
"Exactly. Next?"
-Clue #3: "This is a cropped picture of what appears to be the Diamonds at a costume party, and Blue Diamond's back facing the camera, and...Oh wait. This is an album cover from my record collection.
-Clue #3: "Here's Pink in a Shakespeare-in-the-Park™ production of Romeo and Juliet. She played Juliet. Guess who dies in the end."
-Clue #4: "Leave it to Pink Diamond to invent the 'selfie'. Here she is at the Zoo's landing bay, with a new batch of humans. The hatchway has a serial number that reads SAT6. Now, think about it, SATurday the 6th, the day she was shattered. I rest my case."
Steven held Laser Pointer!Amethyst off to the side and triumphantly dropped her. Amethyst shifted back to normal and ran off to join the others. Steven turned toward the tape recorder and, pressing PLAY, continued his report.
-Clue #5: "This is a black box recording from the day Pink went to see the Diamonds to plead...no...beg them to spare the Earth. If you turn up the volume and slow down the tape, you can hear someone...or something whisper 'I shattered Pink' under their breath, yet audible enough to be picked up by the microphone.
-Clue #6: "This is an excerpt from one of Pink's holiday speeches to the inhabitants of her Zoo. Nearing the end, you can hear Pearl, who was standing off to the side, ask her 'How do you turn this off again?'. A naked ear would be able to pick up 'turn', 'off', and 'again'. But if you play the tape backwards, in reverse, 'How do you turn this off again?' sounds a lot like 'Pink is gone for good'.
-Clue #7: "Another one from the black box, this is Yellow Diamond calling for Pink on a PA system. Again, if you run it the other way, it goes from 'Pink Diamond to the throne room? Pink Diamond to the throne room?' to 'More for me, dust pile. More for me, dust pile'."
"Well?" said Steven as he finished his investigation.
The Yellow Diamond didn't know what to say, but she didn't seem pleased.
"Do you really expect me to play along with one silly superstition after another?" she scolded.
"Wait!" said the Blue Diamond, "I think 'Steven' is right. Pink wanted us to know that she was leaving!"
"See? Blue gets it!" said Steven, "And this probably wouldn't have happened if you listened to Pink Diamond and left the Earth alone."
There was a long pause.
"Eh?...Right?"
The Blue Diamond began to sob again like there's no tomorrow. The Yellow and White Diamonds walked over to comfort her. The White Diamond turned to Steven.
"I think you'd all better leave," she said, "We'll meet you later at the ship. Maybe."
Steven did what was best for the situation, and left with the others. Who know the Blue Diamond was capable of getting this emotional?