Nearly every diamond contains some external blemishes or internal inclusions that can be detected with magnification. To the extent that diamonds are free of these characteristics they are more rare and valuable. Diamonds with higher clarity or "purity" are more expensive than diamonds with lower clarity and more imperfections. In addition to being rarer, stones with higher clarity are capable of reflecting and refracting light better.

Diamonds are graded for clarity based upon the number, size and location of these tiny inclusions. They also serve to create a very unique identifying signature for a specific diamond, much like a fingerprint.

The clarity grading system most widely used around the world is the GIA scale. Professional grading is done in controlled conditions under 10X magnification.

It is helpful to think of clarity in terms of grade ranges with the following descriptions.

Clarity Descriptions
Abbreviation	Definition	Description
F, IF	Flawless - Internally Flawless	Flawless stones have no external blemishes or internal inclusions when examined by an experienced grader using 10X magnification. Internally Flawless diamonds have no inclusions under 10X magnification but have minute external blemishes.
VVS1, VVS2	Very, Very Slightly Include	Contains minute inclusions that are difficult even for an experienced grader to see under 10X magnification.
VS1, VS2	Very Slightly Included	Contains minor inclusions observed with effort under 10X magnification.
SI1, SI2, *Si3 (not used by GIA)	Slightly Included	Contain inclusions that are noticeable to an experienced grader under 10X magnification. Almost always clean to the naked eye.
I1, I2, I3	Included	Contain inclusions obvious under 10X magnification. Inclusions may affect transparency, brilliance and even durability.

Practical advice:

For jewelry purposes Si2 clarity is sufficient to allow for brilliance and performance. Even close examination with the naked eye will not reveal any flaws in most cases. For engagement purposes many buyers prefer stones with higher purity. Selecting Si1 or Si2 rather than a higher grade can enable a buyer to own a larger and/or better color diamond for the money without a substantial drop off in beauty or performance.