When we speak of a diamond's clarity, we are referring to the presence of identifying characteristics on (blemishes) and within (inclusions) the stone.
If you think about the incredible amount of pressure it takes to create a diamond and the fact that natural diamonds are not grown in a sterile laboratory, it's no surprise that most diamonds have flaws. For clarity grades F through SI, inclusions (internal flaws) are NOT visible to the naked eye.
Basically there are two types of flaws: inclusions and blemishes. Inclusions refer to internal flaws and blemishes refer to surface flaws. However, in the diamond grades listed below, you'll note that none of the grades include the term "blemish" -- for the purposes of grading diamonds, all flaws are called "inclusions."
Inclusions include flaws such as air bubbles, cracks, and non-diamond minerals found in the diamond. Blemishes include scratches, pits, and chips. Some blemishes occur during the cutting processes (most often at the girdle). Diamonds with no or few inclusions and blemishes are more highly valued than those with less clarity because they are rarer.
How are diamonds graded for clarity?
Diamonds are graded for clarity under 10x loupe magnification. Grades range from Flawless (diamonds which are completely free of blemishes and inclusions), to Included 3 (diamonds which possess large, heavy blemishes and inclusions that are visible to the naked eye).
Flawless: No internal or external flaws. Extremely rare.
Internally Flawless: no internal flaws, but some surface flaws. Very rare.
Very Very Slightly Included (two grades). Minute inclusions very difficult to detect under 10x magnification by a trained gemologist.
Very Slightly Included (two grades). Minute inclusions seen only with difficulty under 10x magnification.
Slightly Included (two grades). Minute inclusions more easily detected under 10x magnification.
Included (three grades). Inclusions visible under 10x magnification AS WELL AS to the human eye. We do not recommend buying diamonds in any of these grades.
While the presence of these clarity characteristics (inclusions and blemishes) do lower the clarity grade of a diamond, they can also be viewed as proof of a diamond's identity. GIA certificates include what is known as a "plot" of a diamond's inclusions -- think of it as a "diamond fingerprint." Since no two diamonds are exactly the same, comparing the uniqueness of your diamond's clarity characteristics with the plot provided on the diamond certificate offers assurance that the diamond you pay for is the same diamond you receive.
Which Clarity Grade Should I Choose?
While Flawless diamonds are the rarest, a diamond does not have to be flawless to be stunning.
In fact, until you drop to the "I" grade, a diamond's clarity grade has an impact on the diamond's value, not on the unmagnified diamond's appearance.
Diamonds with VVS and VS grades are excellent choices for both value and appearance.
More affordable (and still a great choice) are those diamonds which gemologists call "eye-clean" - diamonds with no inclusions visible to the naked eye. These diamonds are SI1 and SI2 and unless the recipient carries a 10X loupe (a strong jewelry magnifying glass), she won't see the inclusions.
As to I1-I3? Maybe when there's a diamond grade that's defined as "you can see the flaws just by looking at the diamond," nothing more needs to be said.
But if you choose to buy an I1-I3 diamond (which we don't really recommend), know that some people will look at it and immediately see the flaws -- and not just experienced jewelers.
This right up is from the GIA web site.