Every diamond has what we call natural inclusions or imperfections of some sort. So we have to have a cut off line of some sort to define the degree of these inclusions. With modern technology, we can magnify a stone to many hundreds of magnifications, so if the practised diamond grader cannot determine any internal inclusions of any sort under 10 X magnification, the stone is considered “Pure” or “Flawless”.
As the degree of imperfections increases so titles are assigned to the diamond.
FL (Flawless) VVS, (very, very small inclusions) VS (very small inclusion) SI (small inclusion) or I (Included or imperfect)
As we drop down the perfection list, so the stone becomes less valuable.
Why? Because there are fewer flawless diamonds than Imperfect diamonds.