Here is a synopsis of a very useful and informative seminar given by Diane Flora, CGA, which describes the grading techniques used by GIA’s Gem Trade Laboratory. Flora notes five factors determine a clarity grade: Size, number, position, nature and relief. No one factor is more important than the other, and each will have different degrees of importance to each diamond.
“Size” means the size of the inclusion relative to the size of the diamond. The impact of two identically sized inclusions varies depending on the size of the stone. “Number” refers to how many inclusions are in the diamond. A single crystal can reflect multiple times, depending on where it’s located, and therefore must be evaluated as multiple crystals. “Position” can also play a role. If all other factors are equal, inclusions under the table impact a clarity grade more than inclusions under the crown. “Nature” refers to the type of inclusion it is. Crystals and feathers have different impacts on diamonds. If it’s a feather you must consider its size and location, and whether it breaks the stone’s surface. Most feathers do. “Relief”: The color of the inclusion can cause it to stand out in high relief against the background reflections of the diamond. In a near colorless diamond, a black crystal is easier to see than a white one. Evaluate its relief and whether it’s eye-visible. If an inclusion is eye-visible it does not automatically mean a stone is graded “Included.” The other five factors are considered. An IF diamond can have no inclusions and only insignificant blemishes under 10X magnification. If a pinpoint is visible through the pavilion, the diamond usually receives a VVS1, unless the pinpoint is visible through the crowns, and it receives a VVS2. When an inclusion is larger than minute, the clarity grade can typically go no higher than VS1. Below VS1, combinations of factors set the grade. GIA graders often use a technique known as wet grading. Using a wet sponge or brush, with a little soap added, wipe the surface of the diamond to eliminate dust and surface reflections. The soap is needed because plain water beads up on diamonds. After the initial face–up impression of the clarity grade, you check the condition of the culet by looking through the table. A chipped culet prevents a diamond from obtaining an IF grade, because chips are considered inclusions, even though they can sometimes be polished away. The stone is then held table to culet and mentally divided into eight wedges or sections. Each wedge consists of one main pavilion and two lower girdle facets, the girdle in the same area, two upper girdle facets, a bezel and a star facet. The diamond is turned until the grader examines all eight wedges. Higher magnification locates and identifies inclusions, but if it’s not visible at 10-power, it is not a factor in grading. The grader then holds the diamond girdle to girdle, with the pavilion side up, examining it in profile to check the culet. This is to check for any damage or pinpoint inclusions that may have been missed in the initial observations. The diamond is then observed face up at ten power, and given a clarity grade. Then it’s checked, also face up, with a 10-power fully corrected loupe and overall light source. If there are visible inclusions, the grader needs to check the diamond with the unaided eye. Eye-visibility does not set the grade but it can impact the final call.
The Gem Trade Lab can report a diamond as “potential” or “improvable.” A “potential” diamond can be recut to IF without reducing it below a critical weight. For “improvable,” GTL must determine that the clarity grade can be improved, also without recutting it below a critical weight.