Clarity measures the number and size of the tiny imperfections that occur in almost all diamonds. There are 11 clarity grades which start with flawless (FL). Flawless means the diamond has no visible includes or blemishes. Flawless and internally flawless stones are very rare and very expensive. The scale ends at I3: a stone like that comes with inclusions obvious to the naked eye. But most inclusions are very small. Usually even SI classified stones come with inclusions which are not visible to the unaided eye Therefore they can only be seen under magnification. Those Eye-clean diamonds are much less expensive than the FL or the IF.