Peridot is an expert in gem development. She could correlate factors in Amethyst’s hole with her body just from a glance. I understand her not displaying this power with Steven, because he’s human and she doesn’t understand that. But if she had met Rose Quartz, would she have been able to tell something was funky with the lie that she was just a quartz? Would she have been able to tell she was a diamond?
With the gems’ forms being projections, I’d think that Peridot calculated all that by looking at Amythist’s gem, not her body.