Try sending her this article. I'd rather be single than marry someone who has so much more emotion than logic. To put it another way: If the ring is more important to the woman than I am, then she's not marrying me, she's marrying the ring. It's a huge red flag to me.
For the record, not all women insist on a diamond, except reflexively... I actually know few couples that have bought diamond rings (for many reasons - some financial planning (a young couple should be worried about a house, insurance and retirement before a shiny ring) and some ethical (if you buy a new diamond, there is absolutely metaphorical blood on it - terrible juju to kick off a lifetime commitment of love))
Self-interest/selfish alert: if you're rolling in it and get that $30k ring for your spouse-to-be, don't give it to them on their birthday, Christmas or any other gifting occasion, because they don't have to give it back if they decide to ditch you Pre-nuptuals.
In my mind if a marriage isn't a lifetime commitment, then forget any of the other traditions.
In many jurisdictions an engagement ring is considered a conditional gift (conditional upon her agreement to marry you).
This is logical. Otherwise why not string along as many man as possible to the point of proposal, then thank them for the shiny ring and wish them farewell? Take the strategy to its inevitable conclusion, and if the woman in question calls off the engagement at any point, she should return the ring.
Sometimes it's more about understanding what is real and then making a pragmatic choice due to the pain of having to explain one self constantly. A woman could fully understand that a ring is worthless but deal in circles where the public perception is such that having one anyway is beneficial.
