The clarity or purity of diamonds is determined according to the effect that inclusions and marks (imperfections inside the diamond) have on the appearance of the stone when viewed face up at 10x magnification.

To determine the clarity grade of a diamond, the size, colour, number, location and visibility of any internal inclusions or external markings must be taken into account. Inclusions are ranked on a scale of perfection, ranging from Fl (Flawless: no internal or external inclusions visible under 10x magnification) to P (Pique) or I (Included) inclusions visible with the naked eye. If the inclusions are not visible to the naked eye, the diamond is deemed eye clean.

The position of the inclusions is important in choosing a diamond. Some inclusions at or near the edge can be hidden by a mount and therefore have a lesser effect on the beauty of a diamond. An inclusion in the middle or top of a diamond, however, could be more visible and have more impact on its appearance making the diamond less attractive.