A jar of old buttons makes me think of a Nancy Drew story and in it she goes to a rummage sale and buys a jar of buttons. She sorts them out and there is one button that doesn't really look like a button.
It's a mystery of some porcelan doll and when the doll is found the old woman says something about a "jewel" the doll used to wear. Nancy runs home and gets that button and it's the jewel.