CLARITY

Millions of years ago, when the diamond was being created by the intense pressure and heat of the Earth’s interior, traces of microscopic particles were left inside. Other minerals may also be present, or some “fault” may have developed in the structure of the diamond itself. Sometimes, there are even cracks within the diamond. These are, quite simply, geological relics. It means that it is not possible to find two identical diamonds. The fewer inclusions in the diamond, the more valuable it becomes. After all, it is the sparkle from the diamond that we are attracted to, and if there are too many inclusions in the diamond, they break the path of the light, causing the diamond to sparkle less. Under the right conditions, diamonds are the gemstones that sparkle the most. 
Sometimes, a diamond is described as “loupe clean”, i.e., no inclusions are visible at ten times magnification.

Very small inclusions are not visible to the naked eye. On the other hand, when you are taking a close look at diamonds graded I1-I3 and below, it is useful to know that not only do these have many inclusions that disturb the path of the light, but also that they may be brittle and thus break into pieces if struck at the wrong angle.

FL,IF (flawless, internally flawless)

VVS1,VVS2 (very, very small inclusions)

VS1,VS2 (very small inclusions)

SI1,SI2 (small inclusions)
Small inclusions that can be detected with a trained eye and are clearly visible to the untrained eye with a loupe.

Pique I1- I3 (inclusions).
These diamonds have visible inclusions.