I’ve been collecting jewelry inspired by movies and TV shows for years, and I can tell you that some movie producers do collaborate with jewelers to create authentic pieces seen on screen. For example, the iconic ring from 'The Lord of the Rings' was replicated by Jens Hansen, the original designer, and sold as high-quality replicas. While I haven’t seen a specific ladies' gold onyx ring from a movie, brands like Tiffany & Co. have created pieces for films like 'Breakfast at Tiffany’s.' If you’re looking for something similar, check out specialty shops or auctions that deal with movie memorabilia. They often have limited-edition pieces or collaborations that might match what you’re seeking.
Another angle is to look into costume designers who work on films. Sometimes, they commission custom jewelry for characters, and those pieces can later be reproduced for fans. It’s a niche market, but with some digging, you might find what you’re after. I’d recommend searching for jewelry associated with period dramas or fantasy films, as they often feature intricate designs like gold onyx rings.