A diamond certificate verifies each diamond’s specifications, including its color grade, carat weight, clarity grade and cut grade. You should never buy a diamond without a diamond certificate. The most well-known grading laboratories are the Gemological Institute of America (GIA) and the American Gem Society (AGS) and the International Gemological Institute (IGI).
A diamond's cost depends upon objective characteristics popularly known as the 4C's: Color, Clarity, Cut and Carat. The relationship between these four characteristics determines the value of a diamond. Although it is commonly assumed that carat is the most important member of the 4C's, color, cut and clarity have a much greater impact on the appearance of a diamond.