The vast majority of diamonds contain some impurities, known as inclusions, which can detract from their beauty and impede light return effecting that effects the diamond clarity.  The size, number, type and location of these inclusions all factor into the gems final clarity grade. The universal system for grading diamonds is the IF to I3 system, as charted below:

F and IF

F means internally and externally flawless. IF means internally flawless. F and IF diamonds are very rare so are expensive.

 VVS1 and VVS2

VVS means very, very slightly included. This means it is very difficult to see the impurities, even using a 10-power loupe. The numbers represent levels within this grade – VVS1s are cleaner than VVS2s.

 VS1 and VS2

This diamonds clarity is very slightly included, with impurities  not visible to the eye. VVS and VS are very good which means you would need a microscope to see any flaws.

 SI1 and SI2

Diamond clarity on these diamonds, means the inclusions are visible under 10-power magnification and might be visible to the unaided eye.  A diamond is graded SI1 even though it has no more inclusions than an VS2, but because those inclusions occur in the centre of the diamond, they are more noticeable and more likely to block light.

 I1, I2 and I3

These diamonds contain visible inclusions.