Even though the rules for polished diamond grading are very similar throughout the industry, the process towards their standardization is still continuing under the joint effort of the International Diamond Council (IDC) and the World Jewellery Confederation (CIBJO). The efforts to standardize the polished diamond grading are summarized in the latest issue of IDC Rule Book.
When dealing with an already grade diamond, there are many gemological institutes and certification authorities worldwide. The most professional and reputable polished diamond grading organizations in the world are currently the Gemological Institute of America (GIA) and the Hoge Raad voor Diamant (HRD) Antwerp. When learning about diamond grading, the very essential, and frequently described, characteristics are the 4Cs, representing the Color, Clarity, Cut and Carat Weight of a diamond. Below are some of the many useful links to the topic of 4Cs as issued by the above mentioned organizations.
HRD Antwerp: HRD Antwerp - the 4Cs; HRD Antwerp - 4C's Grading Chart (download PDF file)
GIA: GIA - Origin of the Diamond 4Cs; GIA - 4Cs guide (download PDF file)