This is so true.
We are going through my mum's estate at the moment. A few pieces of 'brand' name jewellery have been valued for probate purposes. All worth diddly squat, despite saying "Mappin & Webb" or "Asprey" or "Garrads"
Many of these companies trade on their brand name to hoodwink people into thinking they're buying valuable jewellery.
They're not! OP you'll be parting with cash for the idea you're buying something aspirational and somehow buying something from "Cartier" to make you feel special.
The truth is, most of these brands make money from people like you. Those with a small amount of disposable income who are buying into the idea of the luxury lifestyle. It's what Chanel and Gucci and all the other big brand names do. They make 90% of their revenue from handbags and smaller consumer goods items that make the masses feel the taste of luxury.
The real hard hitters with the money, they can afford the haute couture on the catwalk but they don't make that much money from this line of work. It's all for show though and without it, this marketing exercise, they wouldn't be able to have the mystique of luxury that they go to great lengths to maintain.
I know the jewellery industry now. Too late to advise my DM sadly, I came to it late.
Go to Hatton Garden in central London and find yourself a bespoke jeweller or contact the Goldsmiths Centre for a list of their current goldsmiths. They are all highly trained and extremely capable. Exquisite artists all of them.
I used to know Stuart Paul who created some of the jewellery for the James Bond films, the Love Knot specifically was a big one. I'm not sure if he's still working, it's a long time ago now. He may be retired. He made over 100 of them by hand.
Go to him and he'll make you exactly what you want with hallmarks and it will have a real resale value. And you won't be parting with your cash for the idea of luxury - it will be REAL luxury!
https://www.goldsmiths-centre.org/community/our-craftspeople/stuart-paul-jewellery/