Well...Amethyst never knew Diamond rule. She emerged after the war ended. And Garnet...she would have been shattered just for existing on Homeworld. She nearly was, when she first formed. She's a fusion of two other Gems, Ruby and Sapphire, and different types of Gems aren't permitted to fuse there.

[Not that the same type are permitted to fuse simply out of love, either.

not that she's touched on Pearl at all]