I really love this topic because it's been fascinating to watch my own thinking on it change.
There are many comments below that come from the point of view that diamond rings don't make sense. On one hand this is completely true. On the other hand, whether they do or don't make sense depends on who's evaluating and there are many perspectives one could come at this from.
I wanted to share my point of view and how it evolved in case it helps as nuance (or helps people understand when their partner is coming from)
Growing up really poor and practical, I thought of diamond engagement rings as really stupid and impractical and that I would either never buy one, or buy one begrudgingly.
Fast forward to a few years ago I met the woman that I am now married to. She grew up very differently from me and in many ways we think very differently. For example she values "how things work" and "tradition" much more than I do, because I focus on first principles thinking and how things ought to change.
So her attitude towards diamond rings wasn't "I need something this expensive" or "you'll do it if you love me" but simply "this is how it works."
It would be easy to say that my attitude was more nuanced and evolved but I also recognized that I had dated plenty of women who thought more like I do (in general and perhaps on this topic specifically) but that I didn't want to marry them. That I wanted to marry this woman because of who she is, and her tradition focus is part of that, and her attitude towards diamonds is part of THAT.
I also noticed how women around her felt about their rings. They loved them and thought they were special. Her parents had just celebrated their 40th anniversary and her dad chose to buy her mom a new diamond ring which she loves.
So here I was and my option was to either "put my foot down" and have our engagement go along my original principles or to recognize that my partner by nature of who she is sees it differently and to recognize how much joy and sentiment she will get out of it. I realized it's a no-brainer.
I ended up having fun with it. Visiting old Jewish diamond dealers in dusty and super secured upstairs offices in NYs diamond district. Looking through loupes at the makeup and imperfections until I found a stone that resonated with me.
Long story short I don't regret the approximately 20k I had spent for a second. It has brought more than than worth of joy already. I "get it" more now than I did before.
We can say it's all marketing but everything is marketing, or rather product market fit. We don't really judge people for spending more money (or its equivalent, time) on things that make them happy. If someone spends 20k more on a car because they love it, or have 20k worth of vintage video games in their house, we don't get worked up about it. Those things are marketing too but the product resonated with the audience and I think the diamonds are much the same.
Again it's all about who you are proposing to. The person's attitudes towards things are holistic. If I wanted to marry someone who thought more like I used to, I could have. But turned out the partner I needed was someone who thinks differently - and when you do that you have to accept that and make them happy in a way that makes sense for them.