Diamond
clarity is a measure of the amount of inclusions and the visibility of
these inclusions to the naked eye. The visibilty of these
inclusions depends upon their location as well as their size.
Simply, a diamond with many microscopic inclusions that are well
spaced will gain a better inclusion rating then a diamond with one
large inclusion in the table or top of the diamond.
The ratings for diamond clarity are in descending order:
IF - No inclusions in the diamond.
VVS1/VVS1 - Very very small inclusions not visible to the naked eye.
VS1/VS2 - Very small inclusions still invisible to the naked eye.
SI1/SI2 - Small inclusions, depending on the size and position these are sometimes visible to the naked eye.
PK1/PK2/PK3 - Bad inclusions, looks poor and easily visible to the naked eye.
Below is a video, explaining the basics of diamond clarity, there will be more to come.
This is the second video, in which you can see comparisions between different diamond inclusions, and clarity grades:
