Board Thread:General Discussion/@comment-26596015-20150714032937/@comment-26217701-20150715023219

Morganiteisme wrote: But then why would Amythyst say "I know what shes doing" to steven even though steven saw the same thing as Amythyst the night before

Sticks literally answered that and the show literally explained it.

She  wanted to keep on fusing with Garnet to make Sardonyx because it made her feel strong - the same thing Amethyst did with Garnet, hency why she said she knew. Because she'd been in the same situation.