[bzbarsky]

> Commoners did not have this custom until the twentieth century

It really depends on the culture. Having the groom give the bride something of value is a quite old tradition for Jewish weddings, including "commoners" (read: all Jews, pretty much). A ring was in fact a quite common such gift going back a long way.

That said, if a ring is used in this case it must traditionally be a plain band with no stones; we're not talking diamond rings here.