(From the Four Cs)

Since diamonds are made by nature, many have natural internal blemishes. In fact, only the most rare (and expensive) diamonds are free of inclusions, as they are known. These tiny imperfections were formed by minerals or other Earthly items that became one with the diamond during its birth.

The best way to describe what inclusions look like is to visualize water frozen into a block of pure ice. If there were dust particles in the room or air bubbles when the ice was forming, you would be able see those particles suspended inside the body of the ice.

However, since diamonds are small (relatively speaking) it's very difficult to see inclusions with the naked eye. Therefore, a 10X loupe (a magnifying device) is used to rate clairity.

Many buyers are not concerned with clairity as long as inclusions are not readily visible with the naked eye, which is the case in all diamonds above an I rating. After all, do you expect an admirer to approach you with a 10X loupe and ask to see your ring?

Finally, accent stones are sometimes lower on this scale because their size and placement in a piece makes it difficult to see the inclusions.