Diamond clarity refers to the inclusions (internal features) and blemishes (surface characteristics) of a diamond. The fewer the inclusions, the rarer and more valuable your diamond will be. To determine a diamond's clarity grading the stone is viewed under a 10X magnification. GIA Clarity Scale includes eleven clarity grades from Flawless to I3. #1 Diamond Source sells GIA graded loose diamonds with "SI2" or better clarity.
Small inclusions make every diamond unique and are nature's fingerprints. Generally, they do not affect the beauty or endanger the durability of the diamond. In most cases and depending on diamond size, if a diamond is SI2 or above, the inclusions are not very visible to the unaided eye. Without magnification, you may never see these inclusions. In general, larger diamonds show the inclusions more. Also, inclusions may be more visible with certain cut styles (e.g., "Step Cuts" such as Emerald and Asscher cuts).