The diamond ring has no utility, so there’s no calculation to make. It’s an irrational proof of commitment.
(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.)
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.
I think that's a bridge too far.
There's many clear reasons why people want a diamond, and even an angle like "I want to follow society's expectations" is to me far from being irrational when you're prepping yourself to build a whole life with someone else.
A couple can discuss these matters and come up with alternative paths, people are referencing other materials and I'd include not buying anything fancy and save up for the first payment of their home for instance. But brides requesting a diamond is usually far from lacking rationale.