The origin of diamond engagement rings was insurance: with the decline of "breach of promise" suits, the fiancée at least had something to sell as a settlement if the fiancé reneged on the promise of marriage.
All of that nonsense is long gone; the fetish for engagement rings is just a residual and empty metaphor. Good riddance.
My US gf was astonished and dubious that my relatives (in various other countries) didn't have diamond engagement rings, or engagement rings at all.