Conflict Free Engagement/Wedding Ring Purchase

I am considering asking my girlfriend to marry me sometime in the next few months. We are both passionate about social justice issues and are aware of the role that diamonds can play in funding overseas conflicts (otherwise known as "conflict diamonds" or "blood diamonds"). Though I have seen some websites that sell conflict free diamonds, I am unsure about purchasing from them. Are there any major jewelry store chains that verify that the diamonds they sell are conflict free (as opposed to trusting the word of the national contractors that the jewelry companies buy the diamonds from)? Or are there any smaller companies you recommend that sell conflict-free diamonds? I live in the Chicagoland area.