First things first. No matter what you've heard, read or been told by your best friend in the whole wide world - clarity is NOT a measurement of a diamond's beauty. NO. It's not. Only in very uncommon situations is there any kind of correlation between a diamond's clarity grade and the way it looks to the unaided eye.
With that said, what exactly is a clarity grade? It's a measurement of a diamond's purity and rarity. That's it!
The clarity grade of a diamond is determined by a skilled grader under optimum laboratory conditions using ten-power (10X) magnification. The grader is looking for any characteristics inside the diamond (called inclusions) or any characteristics on the surface (called blemishes). Once the grader has found all these characteristics they are evaluated based on their size, type, position, color and number. Once the grader has finished this process a final clarity grade is assigned. There are eleven clarity grades in the GIA system ranging from Flawless (Fl) to Imperfect 3 (I3).
In a very small percentage of diamonds in the SI range some characteristics may be visible to the unaided eye. Diamonds falling in the Imperfect grades will more commonly have eye-visible inclusions, but an Imperfect grade DOES NOT necessarily mean that the stone will be unattractive to the eye. This is just another reason why is it so important to look at a diamond carefully before you buy it. It's also another reason why you should never buy a diamond based solely on its grades.
Clarity is a range - Many consumers become too fixated on the clarity grade of a stone without considering the bigger picture. Think of a clarity grade as your age. Let's say you're 34 years old (and don't look a day over 29). You could be very close to your 34th birthday (maybe it was yesterday) or you could be really close to your 35th birthday (it could be tomorrow), but when people ask your age you just tell them that you are 34. It works the same way with clarity grades. A VS2 diamond could be high or low in the VS range. In fact, if the diamond at either end of the range was sent to another lab (or even back to the same lab at a later date) it might very well get a different grade. Diamond grading documents don't tell you where in the range a stone falls. It could be right on the edge in either direction or smack in the center of the range. You'll never know. This is a very good reason not to get too hung up on a diamond's clarity grade when making a decision. Always consider beauty first and foremost when selecting a diamond.
Remember - A high clarity grade may mean rarer (and more expensive), but it doesn't mean more beautiful!