Clarity.

This refers  to  the degree to which a diamond is free of interior or exterior blemishes or imperfections. An imperfection may be an impurity or flaw inside the diamond (an inclusion) or a blemish on its surface. Each such imperfection interferes with the passage of light through the stone and lessens the gem’s overall brilliance. The less brilliant the diamond, the less its market value.

The American Gem Society or AGS defines the term flawless as follows:

"The term flawless shall be used to describe a diamond that is free from all internal and external flaws, inclusions or blemishes under skilled observation in normal, natural or artificial light with a ten-power loupe corrected for chromatic and spherical  aberrations;  binocular magnification under dark-field illumination is preferred. No other use of the term flawless, as applied to diamonds, shall be permitted."

The Federal Trade Commission defines the term perfect as follows:

"It is unfair trade practice to use the word perfect, or any other  word, expression or representation of similar import, as descriptive of any diamond that discloses flaws, cracks, carbon-spots, clouds or other blemishes or imperfections of any sort when examined under a ten-power, corrected diamond eye loupe or other equal magnifier."

All diamonds have some internal characteristics. Clarity grading is usually performed under the precision 10x power magnifying glass called a loupe (pronounced loop). Those with characteristics so tiny they can be seen only with a microscope and not with a 10x loupe  are called “flawless”  or Fl on the GIA clarity scale.

The next lowest clarity grades -- VVS (very very slightly included), and VS (very slightly included) -- are given to diamonds with inclusions visible (with difficulty) only with a 10X power jeweler's loupe.

Most better commercial jewelry grade diamonds are of VS or SI (slightly included) clarity. Stones with inclusions visible to the unaided eye are graded I for Imperfect or included. Size and position of the inclusions affect the brilliance and value of a stone according to whether they interfere with the passage of light through the diamond.

While minute inclusions (VVS, VS, SI) reduce the cost of a diamond, they neither mar its beauty nor endanger its durability. However, small internal cracks, known as cleavage, or sometimes referred to as a gletz, although not necessarily visible to the naked eye, may make a diamond more fragile. Some other type of  inclusions are referred to as carbon, feathers and knots.

Let me quickly list  the grading nomenclature   (terminology).

FL  for  internally flawless, the very best grade. No inclusions (flaws) can be seen under 10x jewelers loupe  magnification. 

Next lower grade is VVS-1 and VVS-2 for very very slightly included, level 1 and level 2. These 2 grades are virtually flawless,  You will not be able to see anything at all with 10X jewelers loupe magnification.

Then VS-1 and VS-2 for very slightly included, level 1 and level 2; again these are very fine clarity grades. It is very difficult to find any inclusion with 10X power magnification.

Next SI-1 and SI-2 for slightly included, level 1 and 2. In these, with a little practice and a 10X

jewelers loupe, you can see some small flaws or inclusions.

At the bottom we have SI-3, I-1, I-2,  I-3 for included or inclusions; at I-1 and I-2 you probably won't need any magnification to see some of  the flaws At  I-3 (also called promotional grade or pique) you don't want to get too close or the flaws will  hit you in the eye.

Color    *    Clarity    *    Cut    *    Carat