Sometimes the most economic and heartfelt gesture is DIY
I promise to follow up with more on this later - there are a few services kicking around NYC that cost a pretty penny but allow you to make your own engagement ring. It occurred to me that there had to be workshops available through the fashion schools or a lapsmithing society that would teach a person much more about jewelry design and provide them with the opportunity to make an engagement ring.
So..if you are feeling crafty or want to test an interest in jewelry-making-for-real I wanted to let you know that Fashion Institute of Technology offers continuing education and workshops on jewelry design that would teach you a few of these skills.
For considerably less money, the 92nd street Y offers many different courses in jewelry design - from metal working to stone setting to lost wax work.
For more information on jewerly design courses near you, check out the information page of the Society of American Silversmiths.