As the diamond formation process includes enormous amount of pressure and temperature elevated to thousands degrees, there can be some “inclusions” formed in the structure of the stone made of metals or atom dislocations. The clarity of the diamond depends on the location, amount, size and material of the imperfections. The clearer the diamond seems, the more valuable it is as more light can pass through, creating the sparkle. Heavily included stones, on the other hand, might look plain.
GIA established the following grading criteria of clarity: flawless (FL), internally flawless (IF), very very slightly included (VVS1-VVS2), very slightly included (VS1-VS2), slightly included (SI1-SI2) and included (I1-I3). Flawless and internally flawless diamonds are very pricy, as the inclusions cannot even be noticed with magnification. A better bargain might be an “eye-clean” diamond, which can be graded as SI1 or VS2, though none of the imperfections are visible to a naked eye.
The clarity of the diamond can be improved by the means of laser drilling and fracture filling. It means that if there are serious imperfections in the stone and was graded as included (I1-I3) it could be improved by removing the spots with a laser beam. Such treatment can be seen only with a microscope; however it can weaken the diamond structure. If there are a lot of inclusions removed this way, the stone can be destroyed. Therefore, treated diamonds are less valuable and the treatment should be disclosed to the customer. The same with fracture filament: the scratches can be made less seen, although the sparkle can be decreased due to the material used to cover the imperfections.
The most important thing is to choose a diamond which is eye-clean; it might be VS2 or SI1 clarity. It will not cost as much as FL or IF, however if the inclusions are not severe it might be as sparkly depending on the color and cut of the diamond.