American males enter adulthood through a peculiar rite of passage – they spend most of their savings on a shiny piece of rock. They could invest the money in assets that will compound over time and someday provide a nest egg. Instead, they trade that money for a diamond ring, which isn’t much of an asset at all. As soon as you leave the jeweler with a diamond, it loses over 50% of its value.
Americans exchange diamond rings as part of the engagement process, because in 1938 De Beers decided that they would like us to.