The diamond that you buy may come with an independent grading report (or certificate). There reports give and independent expert opinion on certain aspects of the quality of these diamonds. The grading institutes have labs worldwide and they do not sell diamonds (because they must be independent).
Some well-known organisations that provide grading reports include, but are not limited to, the American Gemological Society (AGS), Diamond High Council (HRD), European Gemological Laboratory (EGL), International Gemological Institute (IGI), and the Gemological Institute of America (GIA).
If you want to buy a diamond with any of these reports, you should go to their website and read about the layout of their reports, and what information you would be able to get.
Accuracy of the reports
Although there is a considerable amount of subjectivity in the grading process, it is generally agreed that AGS, GIA and HRD reports offer the most accurate and consistent information. I would recommend that you pick a diamond with any of these reports.
IGI seems to be getting a lot of bad publicity in internet forums, so I will not recommend diamonds with this certificate.
As for other certificates, they do provide some information during the buying process. However, I would advise you to obtain a gaurantee from the seller and proceed to have the report verified by a qualified appraiser before completing the purchase.
What do grading reports tell you?
In addition to these, a grading report also tells you implicitly that the diamond is not fracture filled, nor colour enhanced because the grading labs do not grade diamonds with these artificial enhancements.
Note that a diamond grading report does not give you the value of the diamond or any other qualities that change with time. The diamond grading report only provides you with information that is permanent (unless you damage the diamond). So, keep your report in a safe place.
The AGS Diamond Quality Document and the GIA Diamond Grading Report provide most of the essential measurements of a diamond's proportions as follows:
You need all these information to determine if the proportion of the diamond is good.
The GIA Gem Trade Lab Report does not provide the crown height, crown angle and pavilion angle of the diamond. However, this does not mean that you should not buy a diamond with the GIA Report because GIA grades their diamonds to very exacting standards. You can always ask your jeweller to provide you with the missing information because they are easily measured with the proper equipment. Alternatively, you can ask the independent appraiser to do it (for a fee).
Cut Grading after January 2006
All labs provide grading information on Polish and Symmetry. However, not all labs give you a grade for cut proportions. That is because it is very difficult to define what exactly is the ideal proportion, and how much tolerance should we allow before it stops being ideal.
The good news for consumers is that after the AGS and GIA have each come up with a largely reliable cut grading methodologies. Therefore, if you have a diamond report that is post January 2006, you can rely on the cut grading, which includes proportions grading. A GIA Excellent or AGS Ideal cut grade would (most likely) be a bright beautiful diamond with no structural integrity issues. However, I would still encourage you to go through the pages on both these gem labs' grading methodologies because they do have their flaws. A diamond is a very expensive purchase and you do not want to rely on the cut grades blindly and thus be saddled with a diamond that is not completely ideal for you. So please go through those important pages.
Does certification increase the value of the diamond?
Certification does not alter the value of the diamond. It is the underlying qualities of the diamond that determines its value. However, it is evident that diamonds with similar characteristics with different certificates command different prices.
Some people argue that the reputable labs serve as a branding tool. For instance, some people feel that diamonds with the AGS Diamond Quality Document is superior to one with the GIA Gem Trade Lab Report, and they are willing to pay more for it.
The other stronger argument is that people are willing to
pay more for diamonds with a report from a lab that has consistently higher
grading standards. They assume that the less reputable lab reports are
softer and factor in the possible difference. For instance, when buying
a 'G' colour diamond with an IGI report, some buyers may think that it
is really only an 'H' colour and price the diamond accordingly.