The author focuses on the "object of beauty" role of the engagement stone, but I think that the more important role in marital tradition is as a "store of value". It publicly indicates the groom's value for the wife, while also handing her a measure of independence, to assist in exiting a bad marriage. That is, it helps to mark the union as consensual.

While a deed to real estate might serve these purposes better than a gemstone, the gemstone is easier to flaunt. But the details of its luster, etc., are still less important to the longevity of the tradition than its economic value.