In this case, challenging the industry means going up against the public's image of what an engagement ring should look like.

It's an image reinforced by years of successful marketing and backed by an enormous monopoly. Some may see that as an unsurmountable challenge, others may see opportunity.

This article posted on HN a few months ago was brilliantly informative:

http://news.ycombinator.com/item?id=4535611

That said, I'd be interested in hearing about the experience as well.