The Clarity of a Diamond is vital to its value and rarity. It is one of the most important C's of "The 4 C's" and refers to the amount (or lack of) inclusions both within the stone and on its surface.

An inclusion is any mark or substance within a gemstone that can be seen by either the unaided eye or with the use of 10x magnification. A Diamond that is free from any inclusions is extremely rare.

A blemish is a mark that ison the surface of the Diamond only. These can include polish lines (extremely hard to see, even for experienced jewellers), burn marks, scratches, etc. In essence, blemishes should be able to be removed with polishing.

Grades - The GIA has developed a grading system for the clarity of Diamonds. Some of the grades have numbers which refer to which end of the spectrum within the grade that the Diamond lies. The grades are as follows and are appointed using 10x magnification only:

Flawless (FL) - void of any inclusions and blemishes

Internally Flawless (IF) - void of any inclusions but has at least one blemish.

Very Very Slightly Included (VVS1/2) - Minute inclusions are visible (usually pits)

Very Slightly Included (VS1/2) - Minor inclusions are visible (usually pits and small crystals that are not in immediate view through the table)

Slightly Included (SI1/2) - Noticeable inclusions are visible (clouds, needles, crystals, etc)

Included (I1/2/3) - Obvious inclusions are visible (an array of all sorts of inclusions can be seen throughout the stone). It should be noted that I3 is a step above an industrial Diamond and is too frail to be safe for an engagement ring.

The more inclusions a Diamond has the less the light can travel through and within the Diamond, thus impacting its intrinsic beauty and overall value and desirability.