Use the same formula that DeBeers uses (but wait, there's more than one!):
http://minuk.vox.com/library/posts/tags/debeers/
http://www.dashes.com/anil/2006/12/01/blood_diamonds
http://minuk.vox.com/library/posts/tags/debeers/
http://www.dashes.com/anil/2006/12/01/blood_diamonds