I think that Amethyst shapeshifts temporarily from her original form to another one, without the need to be poofed. But Pink Diamond was poofed and she reformed as a Rose Quartz. (Amethyst did something like that in the episode Reformed, but Amethyst doesn't have enough power to change her form that drastically this way though)
Well, those are my thoughts. Do you have any ideas about this?
What exactly is happening with him..?