My favorite engagement story is that of my grandparents. He bought her one anyway, but it's the point that she said yes to the marriage request, regardless of the ring. After all, it's all about love, not the material things. And when you get married, start saving slowly for a lovely diamond to pick out together. Set a budget and look for items with no reserve listed.
Most sellers accept money orders.
Check out garage sales. My husband asked his Mom and she gave him a tack pin with a diamond that we brought to a jeweler to have a ring made. If you are really lucky, maybe there is even a wing in waiting. Theresa A Diamond in the Rough Try the pawn shop route. Often you can find an inexpensive diamond or gem in a setting that you like or you can get the ring and have the diamond reset.