Clarity refers to the existence and visual appearance of internal characteristics called inclusions and surface defects called blemishes. These imperfections can affect the relative clarity of the stone and a grade is assigned based on the overall appearance of the stone under 10x magnification.
The diamond grading system today is divided into six categories and eleven grades:
FL Flawless category diamonds that no inclusions or blemishes are visible under 10x magnification.
IF Internally Flawless category diamonds have no inclusions visible under 10x magnification, only small blemishes on the diamond surface.
VVS Very, Very Slightly Included category diamonds have minute inclusions that are difficult for a skilled grader to see under 10x magnification. VVS1 denotes a higher clarity grade than VVS2.
VS Very Slightly Included category diamonds have minor inclusions that are difficult to somewhat easy for a trained grader to see when viewed under 10x magnification. VS1 denotes a higher clarity grade than VS2.
SI Slightly Included category diamonds have noticeable inclusions that are easy to very easy for a trained grader to see when viewed under 10x magnification. The SI category is divided into two grades; SI1 denotes a higher clarity grade than SI2. These may or may not be noticeable to the naked eye.
I Included category diamonds have obvious inclusions that are clearly visible to a trained grader under 10x magnification. The I category is divided into three grades; I1 denotes a higher clarity grade than I2, which in turn is higher than I3. Inclusions in I1 diamonds often are seen to the unaided eye. I2 inclusions are easily seen, while I3 diamonds have large and extremely easy to see inclusions that typically impact the brilliance of the diamond.
A gemstone's clarity grade is directly related to its relative freedom from clarity characteristics. These include inclusions, which lie within the stone, or blemishes, which lie on the surface of a gem. The fewer clarity characteristics, the more valuable the gemstone.
Each variety of gemstone has its own clarity standards. For example, Tanzanite is virtually inclusion-free, while Emerald almost always contains clarity characteristics. For this reason, gemstone grading divides transparent coloured gemstones into three clarity types. This allows gems to be more evenly evaluated as it takes into account the individual nature of each gemstone.
Type I gems are typically free of clarity characteristics. Most commonly these include green tourmaline, tanzanite and blue topaz.
Type II gems usually contain some inclusions and/or blemishes. These include alexandrite, garnet, smoky quartz, amethyst, iolite, sapphire and ruby.
Type III gems like emerald and watermelon tourmaline almost always contain clarity characteristics.