It is a good idea to buy a diamond with an independent grading report. Grading reports do not give a value; appraisals do that. They give an independent expert opinion on quality for loose or unset gems. Most grading institutes have labs or services worldwide and do not sell diamonds. There can be slight differences in the grades and prices of the same diamonds graded and priced by different labs.
 
More GIA diamonds are listed on the internet. IGI and EGL certified diamonds are common in stores; these labs bundle appraisal and grading services together. Appraisals go out of date, but unless the diamond is damaged, the cert is good forever. Store it in a safe place.