The engagement ring is useful when there is an asymmetric relationship in the earning potential of the proposer and proposee, and simultaneously a potential irrecoverable cost on the part of the proposee from agreeing to marriage. (Historically, high wager earner male, and child bearing years female - this historical antecedent has less relevance in contemporary times) In order to fulfill their economic role in proposing marriage, the engagement ring (or whatever token is used) needs to have two properties:

o It needs to be scale ably expensive, that is, there needs to be a way for someone who is making any range of income to find something that is "expensive" for them.

o It needs to have little to no resale value.

This allows the potential proposee to ensure that they aren't just one of many target (it would be expensive for the proposer to offer this token to many people) demonstrating long term commitment on the part of the proposer, and, at the same time, protects the proposer from "gold diggers" - the token, (e.g. Diamond Ring) - has little to no resale value.

Should Diamonds no longer fulfill that role, then other tokens may arise - I've often wondered whether expensive / lavish wedding parties could server this purpose.