There's been many opportunities now for the girl to explain herself, or for the others to press on about what she meant, but the author thinks whatever the reveal he's prepared where the girl goes "Aha! That's why this necklace is considered cursed!" is well worth the minor contrivances along the way
we're told why she doesn't say anything in this very chapter.
Because all her life, she's tried to explain herself, but because she can't always put it into words well (smells like autism/spectrum stuff to me), she's endured a lifetime of hardship of people not believing her, to the point of spreading baseless rumors and vilifying and persecuting her and by extension her family.
She's come to simply not trust that anyone would actually listen to her. And she was trying to speak of danger to
a queen of a foreign, adversarial power, while being a guest on her own with no support base beyond a prince who was acting purely on ulterior motives in bringing her over in the first place.
And she found out that the same "gem-eating villainess" rumors had made it all the way to this empire, as well.
It's entirely reasonable as to why she didn't actually speak out. Experience has shown her again and again that most people wouldn't listen, much less understand, and she would simply be further hated and reviled as a result, only this time by a
second set of literal royalty.
And--she still doesn't trust the prince, because she has been conditioned to believe that
no one will actually trust her, and that she's simply here under the terms that she assists him with the counterfeit gemstone issue he expressed. Everything in her life up to this point has shown her that no one actually cares about her and that getting her hopes up that "at least one person finally sees her for her", is a fool's errand.
This is arguably pretty well-written up to this point, given that setup.