The clarity of a diamond refers to the presence (or lack) of imperfections.
Generally, the more imperfections (or inclusions) a diamond has, the lower the clarity. But number of inclusions is not the only measure of diamond clarity. Other inclusion factors that affect clarity include:
Size - How large the inclusion is.
Number - How many inclusions are present
Position - Where the inclusion is located. For instance, it may be more hidden in the inside lower facets, or more obvious on the surface of the diamond.
Nature - Based on their size and location some inclusions may affect the durability of the diamond.
Relief - Based on color, some inclusions may be more apparent than others. For example black inclusions are much more visible than white or clear inclusions.
The American Gem Society (AGS) has a scale to measure diamond clarity, ranging from 0 (flawless) to 10 (included).
Flawless - No inclusions or surface blemishes under 10X magnification. Internally flawless - No inclusions under 10X magnification; either no surface blemishes, or insignificant surface blemishes that can be removed with minor polishing.
Very, very slightly included - Diamonds will have minor inclusion or blemishes that are difficult for even a trained gemologist to see.
Very slightly included - Minor inclusions or blemishes that are generally not visible to the naked eye. These inclusions are somewhat difficult to see with a trained eye and would be very difficult to see with an untrained eye under 10X magnification.
Slightly included - Inclusions that are easy for a trained eye to see under 10X magnification. These inclusions may be visible to the naked eye, although they are generally difficult to see with an untrained eye in stones under 1ct.
I1, I2, I3
Included - Inclusions that are very easy for a trained eye to see under 10X magnification. These inclusions are often visible to the naked eye and in the case of I3 diamonds the inclusions may affect the structural integrity of the stone as well as impacting its brilliance.