Some of the most valued diamonds are the ones with the fewest natural flaws or inclusions.
The grading of diamond clarity is based on the number, size, location, and type of the inclusions found in a diamond.
 

FL, IF             -   Flawless diamonds with no inclusions whatsoever. These are very

                          rare and magnificent diamonds.

VVS1, VVS2  -   Diamonds with very, very small inclusions which are difficult to

                         detect even under 10x magnification.

VS1, VS2        -  Diamonds with very small inclusions that are not visible to the unaided eye.
 

SI1, SI2           -  Diamonds with small inclusions or flaws that may be visible to the

                          unaided eye.

I1, I2, I3          -  Diamonds with inclusions that are visible without magnification.