When you buy jewelry, you'll see statements about GIA certification. GIA is the Gemological Institute of America, one of the premier gemological institutions and laboratories in the world. They offer a certification in which the diamond is graded on specific features, and in fact, you may hear "certified" and "graded" used in the same manner. The certification and the report you get show that the diamond is authentic, and you can have colored stones graded, too. If you're merely buying a diamond, you want to go to jewelers who have already had the diamond certified.
A Grading Report
When a diamond is GIA-certified, it has a report that shows all the details about the diamond, from facet angle (in some cases) to color grade to clarity and more. Certification is about more than whether the diamond is real. It's a description of everything about the diamond to help identify it and give you an idea of what it could be worth (although they don't appraise it or give you a value; more on that in a bit). If you go to a seller who is offering you diamonds that aren't graded, you have to rely on the seller being honest about their goods. With a GIA report, you know what you're buying.
Information for Appraisal
The information included in a GIA report does not include appraising; GIA will not give you a dollar value for the diamond. However, the report contains so much detail about the diamond that it can be used for an appraisal along with a physical inspection. It's also a good way to see if an appraiser is being accurate. GIA certification follows stringent rules, and what's on the report should be true. If you buy a GIA-certified diamond from a jeweler and then get the report from GIA (you can look up the report number on their website), you can learn more about the diamond you bought and, later on, if you want to sell the diamond again, you can use the report to help the appraiser.
Optional Serial Number Engraving
GIA certification does not automatically include engraving the serial number of the report on the diamond. However, if you have a diamond graded, you can request it; the number will be engraved along the girdle, or that thin strip that separates the top from the bottom of the diamond. If you're buying a diamond from a jeweler who had the diamond graded, ask if they had the serial number engraved on the diamond. When you see that engraving, you can find the GIA report. Note that these inscriptions are tiny and generally have to be read with magnification, such as under a microscope.
For more information on GIA-certified diamonds, contact a professional near you.