"It's because of how they make new Gems," he explains, sounding a little bit extra tired as he does so. This is something that still isn't fully resolved. "They have to... inject this stuff into the ground, on a planet that supports life. Because for a Gem to form, it needs to be able to pull that energy out of the world around it. Places that have produced too many Gems stop supporting life, and nothing can fix that..."

It really sucks.

"So when my mom first showed up on Earth, they were just trying to expand their empire. Gems don't really think of other species as... important. They were going to kill the Earth just to make more of themselves. Once Homeworld was chased off, they started wanting to destroy the Earth for revenge, instead."

At least, it seemed like that's what Yellow Diamond wanted to do. Destroy the Earth mostly so she wouldn't have to think about it anymore.