Rey isn't sure if it's better or worse that Qui-Gon doesn't seem offended by her obvious distrust. On one hand, he isn't trying to justify himself or going overboard with attempts to convince her he's trustworthy, which would be suspicious. But on the other hand, he seems to have no problem with her wariness, almost like he anticipates it. Which is also questionable.

But ultimately she decides there can't be any harm in trying this one thing... and then afterward she'll deal with what sort of trick the Jedi has up his sleeves. He's already agreed to just one rock, so if she actually succeeds, at least he can't start adding more to their bargain.

Rey opens her hand, letting go of her physical grip on the rock she's been holding while grasping it again with the Force. Qui-Gon probably meant for her to use one of the stones he left in a pile, but if so then it's his own fault for not specifying. She knows she can move this one, so she's using it. So there. Counter to Qui-Gon's demonstration, Rey keeps her eyes open to watch the rock follow where her hand guides it, and her brow is heavily furrowed in concentration. But getting it over to the pile the Jedi made was never really the hardest part of this, though it's still certainly a challenge to keep the dumb thing from shooting toward or away from her. No, no, the most difficult part is doing his stupid balancing trick. Dumb Jedi parlor tricks. Stupid Jedi tricking-her-into-training-again rubbish. Nonetheless, after a fair amount of frowning and scrunching up her nose in a near-snarl, the rock does eventually perch on its corner on top of another and stays for two whole seconds before toppling over.