Updated: Apr 22, 2021
Next on the list is clarity. Almost all diamonds will have some type of inclusion inside, the severity of which will determine its clarity grade. Clarity grades and definitions are as follows: Flawless/Internally Flawless: No inclusions Internally Flawless may have a natural (skin of the original diamond crystal left by the cutter) or something on the surface, but nothing internally. If you are looking for perfection, this is it. VVS1/VVS2: Very, Very Slightly Included: Minute inclusions Very, very hard to find the inclusions even at 10 power magnification. Absolutely, positively eye-clean; in other words, you will not be able to see anything in these stones with your eye, and probably not even with the microscope (or loupe). VS1/VS2: Very Slightly Included: Minor Inclusions Hard to find the inclusions at 10 power magnification. Eye-clean from the face up position (you might be able to see some inclusions from the pavilion with your eye in a VS2) SI1/SI2: Slightly Included: Small Inclusions Easy to find inclusions at 10 power magnification. Often eye-clean, especially the SI1 quality stones. This is where the vast majority of stones will fall, especially those from fine jewelers. I sold more SI quality stones in my career than anything else because the value is so great. Clarity is important, but if the stone is eye-clean, no one except you, your jeweler and your appraiser is going to know what the inclusions look like! I1/I2/I3- Imperfect: Obvious Inclusions These stones have noticeable inclusions to the unaided eye. There is also a raging debate over a designation called SI3. It is included on the chart above, and is a grade you might see on a price tag, so it is something I have included. It is what some consider a "Low SI2 or a High I1", so that's why there are those who use it. As someone who sticks to GIA standards, it is not a grade I give; however, if you have purchased something as an SI3 and I grade is an I1, know that those are essentially synonymous.
When you have a stone in the Imperfect category, be aware that there is a higher chance of pressure-sensitive inclusions in Imperfect stones; that is, there's a chance the diamond may break under the pressure of setting. The more inclusions, especially feathers (breaks within the crystal structure of the stone), the more chance of the diamond not making it through the setting process. Then again, all diamonds are crystalline regardless of the clarity grade, so there is always that chance.
As always, feel free to ask me if you have any questions!