What You Need to Know About GIA Certified Diamonds. A GIA certification is issued by an independent gemological laboratory. The certificate, also known as a Diamond Grading Report, includes the carat weight, as well as the measurements and grades for the cut, colour and clarity of the stone. These grades are based on GIA’s lab grading standards, which is the most recognized grading structure in the world for diamonds. The certificate also includes the symmetry, polish and fluorescence grades of the diamond. By choosing a GIA graded diamond, you ensure that the quality and specifications of the stone are properly graded without a biased opinion. You also ensure that your stone will retain its value over time, giving you the highest value when looking to sell or replace.