Board Thread:Ask the Staff/@comment-24747384-20160806192238/@comment-26198169-20160806203411

The thing is that we don't really know when Gems arrived on Earth, just when the Rebellion began. She could've easily been made on Earth very early on, then brought to Homeworld, due to how special she was, and then brought back to Earth where she truly saw the damage Gemkind was causing to the planet. This, accompanied by her dislike for the set roles by Homeworld for Gems, caused her to defect and start the rebellion.

That's what I think at least.