My personal theory is that Pink Diamond was shattered by White Diamond, Yellow Diamond, or both.

Perhaps Pink Diamond was trying to negotiate a ceasefire with the Gem Rebellion, or maybe even considering joining their side against Homeworld. For the former, Yellow/White Diamond caught wind of this, and because the Diamonds are strict enforcers of restraining the free will of the other gems, she/they thought that not snuffing out the Rebellion entirely would be a bad idea, so she/they decided to assassinate Pink Diamond and pin it on the Rebellion to make the rebels look bad.

Blue Diamond is clearly the most empathic and reasonable of the Yellow/Blue duo, asking for a full story rather than trying to "get the trial over with" and actually listening to Defense Zircon's theory. And because she constantly grieves for Pink Diamond, her being a suspect is unlikely.

Yellow Diamond, during the trial, is clearly trying to get it over with and hurrying it up. She doesn't act surprised when Zircon accuses her; She straight up zaps her without a second thought, as if she was silencing Zircon to make sure she doesn't say too much. Not to mention in previous episodes, she constantly tells Blue Diamond to shut up and stop moping over Pink Diamond's death. Perhaps she knows more than she lets on…

As for White Diamond, she's the biggest enigma of the whole Diamond Authority, even more than Pink Diamond. Anything goes with her, probably.