The number, position, kind, color and prominence of inclusions contained in the diamond determine the grade of clarity. Most diamonds contain very tiny birthmarks known as inclusions. An inclusion can interfere with the light passing through the diamond. The fewer the inclusions, the more beautiful the diamond will be.
Grades range from flawless (diamonds which are completely free from blemishes and inclusions) to the included 3 (diamonds which possess large, heavy blemishes and inclusions that are visible to the naked eye).