Both can be beautiful. I went with a diamond myself because we both felt it was what we found most traditional and screamed “marriage.” Also I found I fell in love with the look of a very good quality (not necessarily very large) round brilliant diamond, so it was wonderful to be treated to one.
I love colored gemstones too (and wear them on my right hand) but I find them difficult for an engagement ring for a few reasons…
– For rings, durability is an issue. So many gorgeous stones out there, but I worry that their durability just won’t stand up to daily wear. Sapphires should be pretty durable though, and I’m not sure what else.
– Many of them are treated in some way to improve color. Also many of them are synthetic. I don’t know about you, but I only like natural gems, I don’t like them to be artificially grown or “touched up”. A lot of vendors don’t want to give all the details of whether their gems have been treated in any way. That is virtually guaranteed to be a bad sign.
– One very common treatment to improve color is irradiation. This can a) cause the color to be temporary and b) cause the stone itself to become radioactive! I know for a fact that the regulation is sloppy, and jewelers have no way to check and just go on trust from their vendors that their irradiated stones are not “too” radioactive (personally I don’t want ANY radioactivity from it), so unless you plan to check out gems/jewelry with a geiger counter in hand, it’s very, very difficult to know for sure you don’t have something that’s a little x-ray machine on your finger all the time. Not cool with me.
– Their grading is not as developed as diamond grading, less jewelers specialize in them, and pricing is less standardized, so it’s just that much harder to find the stone you want, and then get it assessed, know it’s what you want (like for me, fully natural), and then know you’re getting a good price for it. That’s hard enough to do with diamonds, so it’s really daunting to do in the comparative “wild, wild west” of colored gemstones.
… I hope I didn’t come across too negative. Colored gemstones can be fantastic, there’s just a lot of crap in the way. I’m actually in the market for a natural colored gemstone right now myself (I’m designing a “man-gagement ring” for my FI) that’s why all these issues are so prominent in my mind.