Clarity is affected by internal characteristics called inclusions and surface defects called blemishes. Inclusions might consist of a foreign material, smaller diamond crystals or tiny fractures that cause a cloudy appearance.
The clarity grade of a diamond is determined by a trained technician who examines the stone under 10x magnification.
Inclusions are something like a fingerprint, and can be used to identify a diamond. They also help distinguish a natural diamond from a synthetic or lab-created stone.
Higher clarity is more valued, and contributes to a diamond's light scattering characteristics.
GIA Grading System
|FL||Flawless||Flawless. No inclusions or blemishes visible under 10x magnification.|
|IF||Internally Flawless||Internally flawless. No inclusions visible under 10x, only small blemishes on the surface.|
|VVS||Very, Very Slightly Included||Very, Very Slightly Included. Minute inclusions that are hard to see under 10x. This grade is divided into VVS1 and VVS2, with VVS1 being slightly higher in grade than VVS2.|
|VS||Very Slightly Included||Very Slightly Included. This grade has inclusions that are difficult to somewhat easy to see under 10x. Again, this grade is subdivided into VS1 and VS2.|
|SI||Slightly Included||Slightly Included. This grade has inclusions that are easy to very easy to see under 10x magnification, and is divided into SI1 and SI2.|
|I||Included||Included. This grade has inclusions that are easily visible under 10x, and are usually visible to the naked eye. This grade is subdivided into I1, I2 and I3. I1 inclusions can sometimes be seen by the naked eye, while I2 inclusions are easily seen unaided. I3 diamonds have large, easily seen inclusions that affect the brilliance of the stone and may even affect its structure.|
American Gem Society Grading (AGS)
When grading for clarity, the grader considers 5 factors: size of inclusions, number of inclusions, position, nature and color. The grade is set according to the characteristics of the most obvious inclusions, which are also called "grade setting inclusions".
The position of the inclusion is important because it is more apparent in some positions than others. Inclusions are most visible when they are located just under the table. If such an inclusion is also located close to a pavilion facet, it will be reflected many times all around the stone, and is called a "reflector". In this case, each reflection is regarded as if it was another inclusion. Inclusions are less visible when located under crown facets or near the girdle of the diamond.
Only 20 percent of diamonds have clarity high enough to use in jewelry, the rest are used in industrial applications.
FL and IF diamonds are also referred to as "museum quality" or "investment grade". These terms are more marketing hype than anything, as they cannot all be in museums, and are not generally kept as investments.
Diamond clarity can be enhanced by using a laser to burn a hole through to a colored inclusion, then using an acid wash to remove the coloring. The GIA does not certify or grade clarity-enhanced diamonds.
Another enhancement method is to fill cracks to make them less visible. These diamonds are sometimes referred to as "fracture-filled" diamonds. Reputable companies use a filling agent that shows a flash of color, usually orange or pink, in order to improve the appearance without being deceptive. It is important to note that a diamond is fracture-filled on any work orders, because a jeweler working on the setting will have to exercise care not to disturb the filling with heat from his torch. Fillings are not necessarily permanent, and some companies offer repeat treatments if needed.