Diamond clarity relates to the existence and appearance of minature marks within and on the surface of a diamond. All diamonds contain minute flaws and marks, generally traces of their formation. Internal marks are generally called “flaws”, and surface marks are called “blemishes”.
These are graded according to their magnitude, on a scale that runs from flawless through to included.
Below are some photos of actual diamonds, with the inclusions circled. All are E colour.
In a VVS clarity diamond, (magnified 10 times), the inclusion is still very hard to see.
In a VS clarity diamond, the inclusion is still invisible to the naked eye, but can be found using a 10x lens, where it is only a speck.
In an SI clarity diamond, the inclusions are easily spotted under 10x magnification. (Note that the inclusions are reflecting slightly too). Because these wispy inclusions are white, they are less visible than if they were dark, so the diamond is an SI1 clarity rather than an SI2 clarity.
Some more microscope photos of diamonds, of different clarities. The approximate magnification is given for each photo. (The white specks in some of the photos are dust particles!)