The clarity of a diamond is determined by using a scale which contains a combination of letters and numbers to signify the amount and type of inclusions. microscopic cracks, mineral deposits or external markings are all considered inclusions. The Clarity factor is rated  This rating scale ranges from FL to I3, FL meaning "Flawless" and thus the most valuable.

The scale is as follows:

FL:
Diamond free of flaws under 10X magnification.

IF:
Free from internal faults under 10X magnification. These may contain some external miniscule flaws that are so insignificant that they can usually be removed with a simple polishing.

V V S 1:
Very very small inclusions in the stone. Inclusions so small they are difficult to see under 10X magnification. These inclusions must not be on the "Table" field.

V V S 2:
Very very small inclusions anywhere in the stone, only the smallest external defects may be allowed.

VS1:
Only the smallest inclusions are allowable in the field of the table and only small faults elsewhere in the stone.

VS2:
Very small internal faults. There may be very few or small external defects.

SI1:
Small internal faults invisible to the naked eye.

SI2:
Small, easily seen inclusions under magnification on the "Table" field, however still not visible to the naked eye.

I1:
Inclusions easily seen under magnification, and difficult to see with the naked eye. These inclusions do not influence brilliance.

I2:
Large and numerous inclusions, barely visible to the naked eye. These inclusions slightly diminish brilliance.

I3:
Large and numerous inclusions visible to the naked eye that DO dimish brilliance.