Diamonds are graded under 10x Magnification.
Clarity grades are based on the number and location of naturally occurring inclusions within a diamond as seen with no more than 10x magnification.
The most common inclusions are carbon (black spots), feathers & clouds (white areas), and cleavages (small ”cracks”).
Any diamond graded SI2 or above will not have inclusions that you can see with the naked eye. Price increases dramatically with a higher clarity grade.