When diamonds were formed billions of years ago under intense heat and pressure, traces of other gemstones, minerals or gasses may have been trapped inside the diamond material as it cooled. These elements may turn up as tiny specks or crystals that may look like feathers or clouds inside each diamond. Because diamonds formed deep within the earth, under extreme heat and pressure, they often contain unique birthmarks, either internal (inclusions) or external (blemishes). Inclusions and are often referred to as Nature's fingerprints or a diamond's birthmarks; they make each diamond different and unique from the next.

Clarity is also measured by the GIA's grading scale, and the most valuable and rare grade is FL, or flawless. No diamond is absolutely perfect under 10× magnification, though some come close. Known as flawless (FL) diamonds, these are exceptionally rare and most jewelers have never seen one.

Diamond clarity refers to the absence of these inclusions and blemishes. The fewer and less visible these birthmarks are, the more valuable a diamond is. Diamonds without these birthmarks are rare, and rarity affects a diamond’s value. Using the GIA International Diamond Grading System™, diamonds are assigned a clarity grade that ranges from flawless (FL) to diamonds with obvious inclusions (I3).

The GIA Clarity Scale contains 11 grades, with most diamonds falling into the VS (very slightly included) or SI (slightly included) categories. In determining a clarity grade, the GIA system considers the size, nature, position, color or relief, and quantity of clarity characteristics visible under 10× magnification.