Amethyst colored and opal crescent screwbacks

I inherited this from my grandma they were her mothers. They are stunning crescents of possibly amethyst and definitely opal. 3 opals and 4 amethyst each with traditional screwbacks, which make them impossible to wear as i cant seem to get the knack of it. Im curious as to the origins of these if anybody has seen something similair. I have seen some Coro that was alike in the stones and setting but cant be sure as these are not signed. Want to know the history so I can share with my family. This is part of the collection that is never to be sold or go outside the family.