A diamond's clarity is determined by any external irregularities and the number of "inclusions" in the stone--that is, imperfections created by nature when the diamond was formed. Most diamonds do contain some inclusions, formed during their crystallization process, but many are microscopic in nature and can be glimpsed only under powerful magnification. When inclusions do not interfere with the passage of light through the stone, they do not affect its beauty.
The clarity of a diamond is determined by the amount and location of flaws, or blemishes, in the diamond when viewed under 10 power (10x) magnification. The diamonds are rated in clarity from Flawless to Imperfect 3 or I3 (see chart).

Eye Clean Factor (Inclusion visible to the naked eye without much exertion):

  • FL-VS1: 100% GIA certified of diamonds will be eye clean.
  • VS2: 95%
  • SI1: 80%
  • SI2: 50%
  • I1: 20%
  • I2: 5%
It all depends on the nature of the inclusion (black, white etc..) ,location of the inclusion (under the table, off to the side etc..) and how deep in the diamond it is. The cutting style of the diamond also plays a role in the visibility of the blemish. Flaws in an Emerald cut have a bigger chance of being exposed due to it's straight line faceting.

The Certificate is helpful in determining the location and at times the nature of the imperfection but it's best to get an honest opinion from someone with the diamonds on hand (like me :)).
The diagram on the Cert will usually have markings on it showing the nature of the inclusions and where they are located. It does not accurately represent the actual size of the blemish. It should also be noted that due to its 2-dimensional nature, unfortunately we can't discern from the certificate how deep the inclusions are nor the color of the inclusion.