Speaking of US things, what's an engagement ring? :)
And yes, most wedding rings around here are plain gold. I only know one couple with diamonds on their wedding rings and they're kinda nouveau riche and snobby.
We're American, but living in Ireland, and when my wife announced she was engaged her colleagues demanded to see her ring, oddly. Cultural imperialism I suppose.
And I guess most people could not even distinct a diamond from polished glass.