Diamonds are graded on a scale from Flawless to Included. Here is a clarity scale guide for diamond clarity.
Flawless (FL) Most jewelers have never seen a Flawless Diamond. A Flawless Diamond shows no inclusions and are considered very rare. Only 1% of all diamonds in the world are rated as Flawless. Make sure its GIA certified If you plan on purchasing a Flawless diamond.
Inclusion definition:Inclusions are flaws within a diamond, these inclusions form during the diamonds formation process. Here is a chart of inclusion types.
Internally Flawless (IF) Internally Flawless Diamond will never have visible flaws, only under 40x magnification will you be able so spot blemishes and even than you would have to understand from a professional diamond grader or gemologist point of view to spot these flaws. Very Very Slightly Included (VVS1-VS2) VVS or VS describes a diamond that has very slight inclusions. A lot of high quality diamonds fall under the VS1-VS2 category but the slight inclusions are not visible to the naked eye with VS1. VS2 diamonds have inclusions that you might be able to see in have about a 5% chance of seeing inclusions in VS2 Diamonds. VVS1 diamonds fall right under internally flawless.
Slightly Included (SI and SI2) Slightly Included means the diamonds might have slighter inclusions and is noticeable under a 10x magnification by a certified diamond grader.
Included (I1,I2 and I3) Included means that a diamond may be a chipped diamond in which the inclusions are visible to the naked eye under 10x magnification. The inclusions will effect the diamonds clarity but these inclusions are only formed in the process of extreme heat and pressure when the diamond forms in the earths surface or mantle the earths plate tectonics shift or depending on the region or area in which the diamond came from like asteroids for example or into the earths Subduction Zone.
Clarity Conclusion:
Diamonds that have been discovered date back as far as the first plants to ever see the face of this earth. It takes millions of years for a diamond to form under the earths surface and explains why you will also see clarity effect the price that comes with these kinds of inclusions that these natural diamonds form in its formation process.