Clarity is one of the most important characteristics of a diamond. It is the quality of diamonds relating to the existence and visual appearance of internal characteristics of a diamond called inclusions, and surface defects called blemishes.
Basically there are two types of flaws: inclusions and blemishes. Inclusions refer to internal flaws and blemishes refer to surface flaws. Inclusions include flaws such as air bubbles, cracks, and non-diamond minerals found in the diamond. Blemishes include scratches, pits, and chips. Diamonds with no or few inclusions and blemishes are more valuable than those with less clarity.
A clarity grade is assigned based on the overall appearance of the stone under ten times magnification.
The GIA grading system:
F - Flawless: No internal or external flaws. Extremely rare.
IF - Internally Flawless: no internal flaws, but some surface flaws. Very rare.
VVS1-VVS2 - Very Very Slightly Included (two grades): Minute inclusions very difficult to detect under 10x magnification by a trained gemologist.
VS1-VS2 - Very Slightly Included (two grades): Minute inclusions seen only with difficulty under 10x magnification.
SI1-SI2 - Slightly Included (two grades): Minute inclusions more easily detected under 10x magnification.
NOTE: For grades F/IF through SI, a diamond's clarity grade has an impact on the diamond's value, not on the unmagnified diamond's appearance.
I1-I2-I3 - Included (three grades): Inclusions visible under 10x magnification AS WELL AS to the human eye.