I have boxes full of costume jewelry like this. My Grandmother loved this stuff. It may be vintage and some glass costume pieces do have value but its gonna be hard to identify it without a makers mark. I would start by searching the costume jewelry links and try to find a match. Its a tedious job but it may be the only way unless we have a costume jewelry expert here. I used to look in Ruby Lane. https://www.rubylane.com/jewelry?cat=Jewelry:Vintage:Costume
EDIT: Now if you think it could be solid gold or gold filled, take it to a pawn shop and have it tested. Gold is almost always marked. We cannot determine with 100 percent accuracy that its costume or glass but thats what it looks like.
maybe. maybe not. It looks vintage. You have a better chance of selling it if you can identify the maker. Search costume jewelry on eBay to get an idea of price range of this stuff. I sold some of my pieces for a hundred bucks
If the gems glow green under your UV light, they are probably Vaseline glass. That would increase it’s collectibility. To bad it’s missing the bottom gem!