I think more engagement rings will have color to them in the future with the influence of rings like the sapphire royal engagement ring. Diamonds beautiful and traditional but for many people they're just not economical especially when you consider that the wedding will not be cheap either. For me personally I preferred having color to my engagement ring because it seemed more interesting and unique than a diamond that everyone has. Plus, I knew that Mark was definitely limited to a budget. Mark chose the ring himself (with the help of some secret agents) but I definitely dropped hints that I didn't want a diamond.
Check out beautiful examples from Bario-Neal that use colored stones and that I think would be really pretty engagement rings. I like the raw stones and the reticulated bands (kind of bumpy) bands that they have on the site. I'm not suggesting that you buy rings from Bario-Neal (unless you really want to) but get ideas from the site and find a jeweler who can make an affordable version. Definitely not for everyone's taste but it's nice to know that different options are available out there. One thing that I'd like to mention is to keep the wedding band in mind when you think about engagement rings (or mention to your guy if he's choosing the ring). For example, my ring's tapered band is so that a typical wedding won't lay flush to it. It'll require a custom wedding band to match but we didn't discover this problem until we went wedding band shopping. Probably something that should have been pointed out by the jeweler that Mark went to but oh well it doesn't keep me up at night.
[1, top] Custom Turquoise Ring
 Cala Sapphire Ring
 Avens Ruby Ring
 Custom Airy Sapphire Ring
 Custom Bezel Ring with Rough Sapphire
 Custom Rough Emerald Ring