How do they prove that a diamond is conflict-free?
The diamonds are laser-etched with a serial number and a logo. For most Canadian diamonds, the logo is a polar bear (they're mined in the way north) or a maple leaf.
On delivery, you get a certificate with a s/n matching the etched number, with full description of the diamond - size, shape, weight, grading and list of inclusions.
http://www.seattlediamonds.com/diamonds/canadian-diamonds
Edit: also, buying from a known ethical dealer helps, rather than some backdoor place that looks like a set from the movie 'Snatch'.
Kn.
When one person suffers from a delusion, it is called insanity.
When many people suffer from a delusion, it is called religion.