As much as I like this story, the women's fashion is so ugly. I actually liked the red dress better, so the big Cinderella reveal was kinda flat for me (frills, bows, ornate flower accessories AND a large repeated rose pattern?). But that glare could cut diamond! Run hater, run!
I think everyone is right that the gem is a plot piece she recognized. But is there a chance she mad about the present, cuz he said it was a sapphire even though it's green like an emerald? Like he was trying to make her look ignorant and embarrass her? It could just be a translation/author error, though.