Another option: second hand diamonds, which diamond merchants will tell you don't exist. People die, if they don't pass their stones onto their children, this is where they end up.

Sotheby's jewelly auctions. Know what you want, buy stones and ignore settings (they'll be dated and you can sell the metals), and know you're the only retail customer in a room full of diamond merchants.

Another option: get a house deposit and work on building an eventual family with your woman rather than provide her with trinkets.