What is the difference between certified and non-certified diamonds?
A certified diamond is simply one that has been graded by a company whose job is to certify diamonds. A grade then is given to each and every stone according to the four C's. After they grade each stone it receives a certificate that it was graded and the product your purchasing is described gemologically.
But, a non certified diamond is not different from a certified one, except a piece of paper describing it, and your trust has to be in the hand of the salesperson who is selling it to you the consumer.
By law if a diamond has been altered, the store selling these items have to disclose that it is selling altered diamonds. So be a conscientious shopper and do your homework on diamonds, before you set out to purchase a non-certified stone.
Finally, In an effort to verify that diamonds are not 'blood diamonds' -- diamonds gathered as a criminal activity -- there are certificates that purport to show the provenance of a gem, to support the claim that the diamond has been legally mined, cut, polished and traded as a legal gemstone.