Now I just want to preface by saying that I know diamonds are NOT investments and that I'm aware that a multi-decade campaign was launched to basically manipulate women (and their partners) to pay for diamonds as a (false) "investment" in the relationship and/or a sign of commitment. The one month or three months' salary or whatever. Now I love a good sparkly diamond just like many others, and I could see why one might pay cartel prices for them when there's no other choice. Plus, the diamond ring is at least unanimously the woman's property in a divorce, which she could sell to get back on her feet during a time when women were left with nothing without a man. But financially, diamonds are NOT an investment. So now that that's out of the way...
Today, there are MANY choices for an engagement ring, including lab diamonds which are to me superior to natural diamonds aesthetically and ethically. Not judging anyone with natural diamonds -- to each their own. Lab prices have been dropping like crazy. Diamonds that were $3000 just two years ago are now $1000 for equivalent 4Cs, yay! And there's moissanite if you want something sparklier and cheaper and like the concept.
Now my favorite ring that I want to make my engagement ring is art deco-inspired, 18k yellow gold, with a 2 carat vintage cut hexagon lab diamond. It will probably cost like $3000 max. I absolutely don't want to go larger (maybe smaller...), and I absolutely don't believe in spending more than one needs to (even if that's my future husband spending it). But I think the concept of "men should invest in the engagement" should still apply.
What alternative "investments" did your partners do, or could your partner do, for you during an engagement? My bf says that if my ring is cheaper than what he "should" be spending he can just put the "leftover" money in a CD for me or something. Which isn't awfully romantic, but he's pretty practical, and I'm somewhere in the middle. I'm also thinking: paying down more mortgage than expected on a shared home, angel investing for you (under your name), just straight up cash...
He could buy a house and put your and his names on the deed instead of buying a fancy engagement ring. That would be a better choice. That way you could sell the house and give him back his purchase, just like you do with a ring, should you not get married.
There are a few places online that sell second hand diamond jewelry including engagement rings. White diamonds do not retain their resell value so it is a better choice financially to look into the used market. Basically, you can get much more blang for your bucks going the consignment route.
Natural yellow, pink, blue, etc diamonds appreciate in value.
I sold my wedding and engagement rings and got $500 when I desperately needed it. Certainly not what they were originally worth, but it helped a ton. I think the ring should cost at least 2 paychecks. It shows the man planned for it and saved for it.