During the formation of a diamond it is possible for minute particles of non-crystallised carbon or non-diamond crystals to be caught within the diamond. These imperfections are called inclusions and provide each individual diamond with unique characteristics. Inclusions may not always be visible to the naked eye; but if they are large enough they will interfere with the passage of the light through the diamond.

Therefore the fewer inclusions a diamond has, the more valuable it is. Like colour, clarity is also categorised using international grading scales. The categories of clarity are based upon the number, size and position of the inclusions within the diamond. Gradings range from flawless, and internally flawless, through very small and small inclusions, to imperfect.