geminigenitalia:

If this really was Lapis, which it seems it is, and she was part of BDs court, then she had exposure to the rebellion group in its infancy, while it was only RQ and P. Lapis probably heard reports of it’s growth with the addition of Garnet. She might have felt betrayed that a Homeworld gem, and an aristocrat at that (I don’t think she’d really care about Ruby since they’re considered to be common soldiers), betrayed her own people and ideals. And eventually, when they gained Amethyst, she could have been sent to attack the group by BD. Maybe Rose sealed Lapis in the mirror, and that’s why the others don’t know how it happened? I don’t really want to think they’re lying to Steven. Especially with something that serious.

Well if you look at the reactions to the gems when they find out about lapis

Garnet is angry while pearl and amethyst are shocked

Garnet gets more angry and amethyst looks scared… or guilty… Pearl on the other hand looks congused more than anything else.

I think that lapis wasn’t sealed until after amethyst joined the group and pearl had no idea. But amethyst and garnet knew. And so did rose probably.