Clarity is a gemstones relative freedom from clarity characteristics, which are classified as inclusions and blemishes. Inclusions are enclosed within a diamond, or they extend into the diamond from its surface. Blemishes are confined to the diamond's surface. Inclusions typically have more influence on a diamond's clarity grade than a blemish.
Clarity characteristics make every diamond unique. Its extremely unlikely that two diamonds would have exactly the same clarity characteristics in identical locations. This helps when you have to identify individual diamonds and tell one from the other.
Five factors determine the overall effect of characteristics on a clarity grade. Most of them have to do with how readily you can see the characteristics. The five factors are: Size, Number, Position, Nature, Relief.
The effect of size is obvious: Large inclusions affect clarity more than small ones.
The number of inclusions is also important, but its not just a matter of counting inclusions. A stone can have many tiny inclusions and still be high on the clarity scale. One or two of the largest inclusions usually set the grade.
An inclusion's position affects its visibility. Inclusions are much more visible when they are in the center of the stone than they are near the girdle.
The nature of a diamond's inclusions also influence its clarity grade. Large breaks in the stone, or feathers, are potentially hazardous, especially if they reach the table or extend from the crown through the girdle.
Relief means contrast or visibility. Most inclusions are white or colorless, but some can be black, brown, dark red, or green. The dark inclusions are usually easier to see, so they have a greater impact on the clarity grade than colorless inclusions.