Which diamond retailers provide the customer with a "certificate of origin"?

Question

I understand that a certificate of origin guarantees that the diamond was not retrieved from a conflict area. I am soon to be engaged and we do not want to fuel the horrific tragedies of African warfare. We are looking for a reputable diamond retailer who can prove the origin of their diamonds. Please include references so that we may research all findings.

Answer 1

Sorry I only shop for "blood" diamonds

Answer 2

I believe any retailer will if you request it.  There are very few diamonds that don't come from Africa though.