OK so to all SU fans out there: most of us watched the episode called "The Trail", right? well then, we should all know that Blue Zircon said that Yellow could've shattered Pink. and I'm standing with Steven. (I may not be in the show or on homeworld, but whatever. I'm in the middle of a roleplay right now with my friend. can't really focus. so sorry if it's scatter minded) so Yellow Diamond has made it clear within many episodes that she shattered someone close. someone with the diamonds gem cut.. and someone... PINK! That's right... Pink Diamond.. Allow me to explain: so yellow diamond may or may not have had a close relationship with her, but Pink Diamond refused to let go of the earth. the result? she shattered her in order to put the Cluster there. (this is what I think. it may or may not be true.) after she shattered her, as seen in message received, she said that she wanted to wipe the earth off their star maps. this reason is unknown. and the reason she wanted to shatter ALL the Rose Quartz' is because some of them likely saw what happened, and she wanted to cover up the evidence. it's unknown where Pink Diamond is now, but I suspect that Yellow Diamond hid the shards to keep them from Blue Diamond. another reason Yellow Diamond might've wanted to shatter Pink Diamond was.... THE SAKE OF LOVE! any of you seen YD x BD? (yellow diamond X blue diamond) well maybe that ship is a reality! as seen in the episodes we saw Blue Diamond, Blue Diamond continues to moarn over her death. (I am SO sorry for your loss, Blue.) So maybe Blue Diamond liked Pink Diamond more than just a friend? and Yellow Diamond liked Blue Diamond? so maybe.. just maybe.. Yellow Diamond shattered Pink Diamond to keep her from winning Blue over! there are many reasons why Yellow would've done it. I've only listed two. I might continue this and say a few more reasons.