Clarity
Diamonds with fewer flaws are rare and therefore more highly prized. In many cases, these flaws (or inclusions) don't detract from the beauty of the diamond because they are invisible to the naked eye. The GIA grading scale below shows clarity ranges from "Internally Flawless" (IF) to "Very Very Slight" Inclusions (VVS), to "Very Slight" Inclusions (VS), to "Slightly Included" (SI), to "Included" (I).
Fast Fact: Any grade "SI2" or above has inclusions that are "eye clean", not visible to the naked eye. An "I1" grade can be an excellent value, particularly if the inclusion is on the outer edge. "I2" grades can still exhibit a fair degree of fire and light. "I3" grade diamonds are noticeably more opaque or dark to the naked eye.