Clarity is one of the "four C's" used to evaluate the quality of gem diamonds (the others being cut, color and carat weight). Clarity is a measure of how many imperfections are visible within a diamond, and it's graded on a scale from "Flawless" on the high end to "Included" on the low end. Diamonds with a clarity grade of I-1 or I-2 are included diamonds.
Diamonds are made up of carbon atoms arranged in a configuration called a "diamond lattice." Unlike other forms of carbon--such as amorphous carbon (common soot) or graphite--the diamond lattice allows all light to pass through, so it appears clear. However, it's rare for a diamond to be perfectly clear because there are usually imperfections in the lattice, such as crystals or small fractures. These imperfections are called inclusions. Their number and placement determine how the diamond is graded for clarity.
The Gemological Institute of America maintains a clarity scale with five levels, with gradations of each level. The top level is "Flawless" diamonds: Those rated FL have no visible flaws whatsoever, while IF diamonds might have the slightest blemishes on the surface but none inside. Below that are "Very, Very Slightly Included" diamonds, with imperfections that are barely visible when magnified 10 times; diamonds in this category are graded VVSI-1 (the higher grade) or VVSI-2. Below that are "Very Slightly Included" (VSI-1 and VSI-2) and "Slightly Included" (SI-1 and SI-2). At the bottom are the "Included" diamonds.
There are three grades of Included diamonds: I-1, I-2 and I-3. There are no hard lines between the grades, but, generally, I-1 and I-2 diamonds have inclusions that are obvious under magnification and visible on close inspection with the naked eye. Diamonds graded I-3 have inclusions that are clearly visible with the naked eye. The more inclusions in a diamond, the lower its "brilliance"--meaning, the less it sparkles.
Common imperfections in a diamond include diamond crystals, "feathers" and "naturals." Diamond crystals are essentially small diamonds trapped within the larger diamond; they're known as "pinpoints," "clouds," "needles" and "knots" based on their characteristics. Feathers are actually small cracks. And naturals are rough patches left on the diamond because removing them would significantly reduce the stone's weight or compromise its structure.
Included diamonds are cheaper than higher grades because of their greater supply and lower desirability. Some diamond dealers, such as Blue Nile, don't even sell them. Some jewelry makers, such as J.C. Hunter Jewelry Design, recommend they be used for earrings and bracelets rather than rings.