So I've beem thinking about what blue Zircon said in the trial recently. Her words ring with truth, but they just dot add up. Though after contemplating the circumstances, I've reached a conclusion. Ladies and gentlemen, I propose that Pink Diamond was shattered by none other than... Yellow Diamond! "But Maximus!" I hear you cry. "You can't just say something like that without anything to back it up" Well don't worry. I have more than enough evidence.
First, let's look at Yellow Diamond's behavior. Yellow Diamond has seemingly been trying to erase the memory of Pink Diamond. She was the one who wanted to shatter all of Pink's remaining soldiers, and she was one one who wanted to destroy Pink's zoo. She even tried to convince Blue to forget all about Pink through form of song. And when you look at the trial, she is constantly trying to hasten the process and skin straight to the execution. And then blue Zircon started to piece things together. She found Yellow Diamond out. But she couldn't have any of that. No. Blue Zircon needed to be silenced immediately, followed by yellow Zircon for good measure.
Now for the factual evidence. While he was on trial, and Steve Universe/Rose Quartz was describing the events that took place on that faithful day, Blue Diamond made it very clear that Rose used her sword to shatter Pink Diamond. However, as we all know, Rose's sword cannot shatter gems, so something is clearly wrong. There's also the matter of how a quartz soldier qould shatter a gem as powerful as a diamond. However, we saw first hand how Yellow Diamond was able to effortlessly poof blue Zircon. I believe that Yellow Diamond arrived at Pink Diamond's palenquin shape shifted as Rose Quartz, and did something that not even Pink's most powerful Sapphires couldn't predict. She shattered one of her own.
And now finally, the motive. What could Yellow Diamond possibly gain from the death of Pink Diamond? She gained everything. First of all, Blue clearly favored Pink over Yellow and White. So if Pink where to be shattered in an act of rebellion, and Yellow where to immediately comfort Blue, she would gain her loyalty. In addition, we now know that the rebellion was started long before Pink's shattering. Yellow did not care at all for them, and wanted the rebellion irradiated. However, they weren't a big enough threat to justify war. She needed some way to escalate the threat of the rebellion so that she could be rid of them once and for all.
And so ladies and gentlemen, I rest my case.
Edited for Spoilers - Slime