With a 10 power diamond loupe, a gauge, the proper light and some experience you may be able to approximate the size and quality of GIA Certified Diamonds.
The first step in manufacturing GIA Certified Diamonds is to give them to a cleaver or sawyer. They both, essentially, perform the same task: That is to cut--or cleave--the antique engagement ring in two pieces in order to bring out the best profile that will determine the end product of the stones.
While nature determines diamond fashion, carat weight and color, the hand of a master craftsman is necessary to release their fire, sparkle and beauty.