I am not sure I buy that reasoning. The ring certainly has utility from the perspective of the giver: the giver knows that it is going to elicit a positive response. A donation to charity cannot be worn, cannot be accessorized, cannot be shown off. I disagree with your assessment - in today's society, a ring has far more social value than a charitable donation.

The ring is shiny, and pretty, and can be socially displayed to elicit the attention of others.

The example you provide re-affirms my point. The evaluation criteria you are reverting to is wealth. And as a heuristic for wealth, the ability to purchase an expensive ring is a reasonable metric.

As a heuristic for a good partner, it is next to useless. As a screening criteria for the complex quality of "goodness" in partner, it is likely less than useless.

If you are evaluating partners purely on their wealth and willingness to waste that wealth to enter into a partnership with you, then a ring is a more reasonable criteria.

For an assessment on more nuanced basis (e.g. does the potential partner truly love me?), it is not a very good indicator, and may even be a contra-indicator precisely because it can be gamed easily.