Earlier quoted context omitted.
The diamond ring has no utility, so there’s no calculation to make. It’s an irrational proof of commitment.
Proof of commitment can have utility. And status symbols can have utility. (Going on a tangent: compare Bitcoin's proof-of-work, which also has utility. Of course, that doesn't mean diamonds nor proof-of-work are the best bang-for-buck. For the latter, you can use proof-of-stake or avoid cryptocurrencies altogether. For the former, there are plenty of other social arrangements that avoid diamond rings.)
Yes, but giving something worthless but expensive is not a real tangible proof of commitment. "Here is something I paid too much for" is less meaningful than "here is something very valuable." Something valuable says if you are bad to her you'll lose out and she will have something to show for the commitment she reciprocated, something expensive that isn't valuable says I'll behave how you want to keep you around.
In India they give gold jewelry, that makes a lot of sense. Their cultural reasoning about it is very different (it's a financial investment that ties families together) but the act is better IMO.