I am preparing to buy an engagement ring.  My question is about the issue of
so-called "conflict diamonds," and associated matters.  In particular, I want
to find out:
   a) what percent of consumer diamonds in the US are conflict diamonds?  
      i.e. if i buy a diamond, how likely is it that it is comes from a conflict
      nation?
   b) are there associated social/political drawbacks with gemstones other
      than diamonds?

Yeah, you want to stay away from those nasty, immoral "conflict
diamonds." For the politically correct, only the morally pure, "used
to finance apartheid diamonds" will do.

If you are thinking about an engagement ring, you might check out
moissanite. It's a US-based manufacturer that has been in the news a
lot lately.
http://www.moissanite.com

You might consider Canadian-mined diamonds as these are guaranteed conflict free.