For example, demonstrate you can care for a dog goes a long way. Owning and maintaining a house even further.
A friend of mine got married. He could've bought his wife an extravagant engagement ring, but he put it to her in different terms, along the lines of: "Would you like an expensive ring, or what about something symbolic and we'll put the rest into a down-payment on a better house?"
They didn't squander tens of thousands on a wedding, nor went on a lavish honeymoon. They started out with money in the bank and a hefty down-payment made on a new house, not credit-card debt.
I really doubt that a significant percentage of those under ten will even bother with a diamond ring when at the age where they'll be getting engaged. By then they'll either be so saddled with student debt as to make the purchase impractical, or they'll be working two minimum wage jobs just to get by.
Fitness indicators change and trend between civilizations. Right now, with the predominance of capitalism, having a lot of "stuff" or access and ability to acquire "stuff" means you're a worthy sexual partner.
It's not sexist or derogatory. Financial security and status are evolutionary concepts that predate the diamond district. Diamonds are just one manifestation of a deeper, hardwired instinct to seek fit and capable mates.
This isn't a cynical concept, it's backed by studies of the entire animal kingdom, not just humans.