Everyone loves a great engagement story and a peek at "the ring" but did you know why we wear engagement rings? According to Stephen J. Dubner (one of the authors of Freakonomics):
"Engagement rings took off in the U.S. when the courts refused to hear “breach of promise” lawsuits. These suits were brought by women who had slept with their fiancés and then been abandoned. These women were then less attractive marriage prospects for anyone else.
Or, if you're into engagement rings...
Naturally, such lawsuits were sensational fun for the newspapers, and eventually the courts put a stop to the whole thing. The problem then became: how could a young affianced couple have sex with each other when she had no recourse to the law if he changed his mind? Both of them might well want to, but for the lady the risks were pretty high. And so the institution of the engagement ring came about. Such rings are non-returnable, meaning that if the man breaks off the engagement he doesn’t get the ring back. The system discourages him from running off and provides automatic compensation if he does. Very clever."
I suspect they aren't terribly useful for that particular purpose these days...but they are shiny, so that's pretty much good enough in my book.