Most Diamonds contains very small traces of material not-crystallized, or they have material not-carbon (Diamond is carbon), that render them virtualy unique.

Generally it is impossible to see these impurities with the nake eye, only the 10x lens, used by the the jeweller, can help us to notice these impurities, that can be white or dark points or asymmetric lines. More a stone is without impurities more is his value, but it is not inevitably the most beautiful stone.

If= Internally flawless, that is nothing visible at the diamondscope
Vvs 1\2= Very very small inclusions, very often they are invisible even with 10x lens
Vs 1\2= Very small inclusions, the stone has inclusions visible with 10x lens, only an expert eye can see them.
Si 1\2\3 = Small inclusions visible with the lens, but not with the naked eye.
P 1\2\3= Pique the stone has inclusions visible with the naked eye (in category 1 only an expert eye can see them)