The grading of diamonds can and does vary. Apart from carat weight (which is not subjective) the color and clarity grades a diamond receives differ, depending on the standards and strictness of the laboratory that does the grading. To a lesser degree grades can even vary within labs.
The American Gem Society (AGS) and Gemological Institute of America (GIA) are recognized as having very strict standards. The International Gemological Institute (IGI) having labs all around the world and the Hoge Raad voor Diamant (HRD) in Antwerp shares a similar reputation for top grading standards. These laboratories hold to the highest, most traditional standards of diamond grading. Other major laboratories vary. In our experience the European Gemological Laboratories USA Group (EGL-USA) has higher standards than their other international locations (EGL-LABS), but all of them are less strict than the AGS and GIA. The world’s most common grading service is the International Gemological Institute (IGI). Many sellers of diamonds, including well-known chains, prefer to have their diamonds graded by the IGI.
All laboratories use trained Graduate Gemologists who adhere to that lab’s chosen set of standards. When grading, inconsistencies may occur when a diamond is on the line between two grades. Each diamond is observed by several graders and the majority opinion is used to issue the final color or clarity grades. The strictest labs are, as a rule, also the most consistent.
Practically every lab provides information on carat weight, grades for color, clarity and finish (polish & symmetry) and girdle & culet measurements. The amount of information provided on a round diamond's cut proportions varies significantly and some laboratories offer different reports internally with varying amounts of information. Most labs, particularly second and third-tier labs, provide no more than depth and table percentage, occasionally supplemented with crown and/or pavilion height percentage. Fancy shapes include even less details as a rule. This is logical, as measurements are far less useful for predicting performance in shapes other than round.
The major laboratories currently reporting all major measurements for round brilliants, including crown and pavilion angle averages, are IGI, AGS and the GIA (although the GIA reports several measurements in rounded form).
There are boutique labs, small and independently run, that provide far more information and reporting than their major counterparts, including in-depth cut evaluation. Notable examples of these are the AGR in Denver, AGA in Philadelphia and SGL in Sarasota.
Cut quality determines how well a diamond will perform, therefore it is paramount in importance, but many grading reports still do not include a grade for cut. The AGS, GIA and IGI again, are the major leaders: The AGS "Diamond Quality Document" (DQD) assigns a grade based on a scan of the diamond which is ray-traced to arrive at numerical values. The AGS also offers a "Diamond Quality Report" (DQR) which costs less and provides all measurements, a cut grade but not light performance. The AGS' top cut grade of 0 ('AGS Ideal') is considered very strict. Since the AGS Ideal cut grade also requires highest marks in proportions factors (durability) and finish (polish & symmetry), these diamonds are sometimes called 'triple zeros.' In 2005 the AGS switched from proportions, or measurements assessment, to direct light performance assessment. The GIA began grading cut for rounds in 2006. Their top cut grade of Excellent is currently based on proportions assessment (much like the old AGS system was). Although they approach cut grading differently, either lab's top grade is a good assurance of durability and finish. The GIA's top grade is not as narrow as the AGS', but the information provided is useful to discerning shoppers.
IGI Antwerp have had a proportion (cut) grade since 1975 - 30 years prior to GIA which has subsequently been improved upon. EGL USA has offerd a performance grade since July 2007 based on Imagem light behavior technology. We will be better able to assess the system's strictness once those diamonds begin hitting the market and reach the hands of independent appraisers and other experts in a position to give unbiased feedback.
Independent appraisers have their own standards for grading. Like laboratories, appraisers vary in strictness and consistency. It is not uncommon for an appraiser doing a blind evaluation to vary from the lab in color or clarity by a grade (particularly when the diamond is on the line between two grades). Mounted diamonds are especially hard to agree-on, so the standard for these may be two grades difference. Some appraisers maintain the high standards of the AGS and GIA while others do not. The AGS GIA and IGI have training programs and most reputable appraisers have completed coursework with these organizations.
Grading accuracy will never be 100% consistent as long as some elements are subjective and graded by humans, but laboratories and appraisers who maintain high standards and disclose complete information also tend to have the greatest consistency and reputability.
For more specific questions ask our experts