So, uh... yeah, we need to talk about A Single Pale Rose.
Here's what I've pieced together; Pink Diamond is a previously unheard of Gem type. She somehow attained Diamond status and was able to live with the other Diamonds. Upon finally getting her own colony, Earth, she fell in love with the planet, and decided to try to keep it from the Diamonds and their plans to destroy it. However, she didn't want to just outright take it for herself and her army turned civilization in the Crystal Gems, so she devised a plan to disguise herself as one of them, and start a rebellion against herself. Somehow, she was able to change the orientation of her Gem to become a completely different Gem type. In order to end things quickly before it got too out of hand, she had Pearl shift into her, while she became Pink Diamond, and was 'shattered' by Rose. Infuriated and terrified at the power Rose had to be able to just shatter a Diamond (which is actually impossible using her sword, as Bismuth explained),
Yellow, White, and Blue decided to flee, but not before combining their powers as light-based beings to corrupt the remaining Crystal Gems by overloading their own light with the combined light of three Diamonds, thus, the Corruption Event. Rose used her shield to save the Gems physically and emotionally closest to her from the photon blast, as seen in Your Mother and Mine. As to how the Cluster factors into this, I think it was put in the planet by the Diamonds when they noticed that Earth was being overrun by Crystal Gems too quickly for them to fight normally. I do believe Pink may be some sort of hybrid experiment thing going on created by the Diamonds and taken in as their daughter.
As for Rose herself,
and so gave up her form as a Diamond and shifted into a normal Rose Quartz. I think that working and exploring Earth as Rose was what caused Pink to fall in love with the planet in the first place. Which is interesting, because the way the Cluster is talked about and the mechanics behind it seem very much like a fetus in a womb, so it may even have been Pink who 'impregnated' the planet she loved with the Cluster, for... some reason. Yeah okay this theory's shit fuck it.