Grading standards of a diamond are cut, color, and clarity.
Cut refers to the angles and polish that determine light's path through the stone, and has the most to do with a diamond's beauty. Cut affects a diamond's value by approximately 50%.
Color has a considerable impact on a diamonds rarity in nature. It refers to the actual body color of the diamond not the color of its sparkle. Colorless is best.
Clarity supports both beauty and rarity. It refers to the small inclusions of carbon, crystals, healed fractures and other materials that may be found within a diamond.
Value standards of a diamond also include carat weight, provenance, and shape and items usually covered under comments. These can include beauty and durability issues.
Larger stones are rarer and in greater demand.
This includes who may have owned it and who graded it. The credibility of a diamond's grade may come from a particular jeweler, independent appraiser, or laboratory which specializes in grading diamonds. Because of slight variations in reputation or criteria, different laboratory reports carry different weight within the trade. Diamonds with reports from some laboratories can demand a premium.
Diamonds are cut into many different shapes. The most popular is the Round Brilliant Cut because it is the most brilliant and has the most potential to display prismatic colors. There are other shapes such as Emerald Cut and Square which can be Step Cut or Princess Cut, Marquise shape (with a point on both ends), Oval, Flanders and Pear Shape (like a tear drop). These cuts require more attention by the buyer to the details of proportion, symmetry and polish than a round shape, as they have the disadvantage of having pavilion angles that vary. This means there is already an amount of light-loss that cannot be avoided. Extra time and care is required to locate a well-cut, fancy-shaped diamond.
Standards for Grading Diamonds
Throughout the world the standard for color and clarity grading is set by the Gemological Institute of America. Laboratories that use another system have comparison charts to the GIA grade so as to be able to be compared. It is not possible to do an accurate comparison without understanding the grades according to these standards. Color starts with the highest grade of D as colorless and goes down through the alphabet. Tints of color become noticeable to the eye in normal lighting at about J in grade. Clarity Uses Flawless, Internally Flawless, VVS (which stands for Very, Very Slightly Included), 1 and 2, VS1, VS2, SI1 to describe the grades of eye clean in an under-carat weight round, brilliant cut diamond. SI2, SI3, I1, I2 and I3 are the remaining grades of gradually more visually affected stones.
The GIA reports have five possible cut grades, and other information needed to determine the quality of cut. The American Gem Society Laboratory grades from 0 to 10, with 0 the most desirable grade for cut and they also grade separately for each of the important aspects. Understanding the comments on these reports is essential to getting a full picture of the value. Having a grade for cut simplifies buying to a degree but it allows a range for the highest grade that still has a few pricing categories within it.