The Gemological Institute of America and the American Gemological Society were both founded by Robert Shipley in the 1930s. GIA’s Gem Trade Laboratory has been grading diamonds since 1939. These organizations are acknowledged as the most reputable diamond grading laboratories in the world. An official GIA or AGS document increases the value of a diamond. As of 2006 both of these labs are assigning cut grades for round brilliant diamonds. The AGS offers a grade for princess cuts as well.
The AGS began as a trade organization focused on ethical standards for retail jewelers and suppliers, and started developing a diamond grading standards manual in the 1960s. In 1996 the AGS Laboratories opened and introduced the first system of grading cut for round brilliants, based on proportions measurements, and officially introduced the term ‘ideal’ to grading. The AGS system is very strict. Only a small percentage of the world’s diamonds can earn the AGS top grade of 0 or “Ideal.” The GIA’s top grade of “Excellent” allows for a broader range of taste and overlaps several AGS grades. A diamond earning one of these grades will have passed all tests for depth, durability, polish and basic symmetry. It will also have visibly superior performance compared to average diamonds.
The AGS Ideal grade is considered very elite. The GIA Excellent grade has more latitude, but is still a good assurance of quality and durability. There are different interpretations of what ‘Ideal’ means. When you are considering a diamond advertised as ‘Ideal’ or ‘Excellent,’ you should find out if it was actually graded by the AGS or the GIA. If so it will be accompanied by an official document. It’s simple to call a diamond ‘ideal,’ but without proof or proportions or performance the term doesn’t guarantee anything.