Most diamonds contain two types of naturally occurring imperfections: internal flaws (inclusions) and surface flaws (blemishes). Taking these traits into consideration, each diamond is given a clarity grade from the GIA's 11-step scale: Flawless (FL), Internally Flawless (IF), Very Very Slightly Included (VVS 1 and VVS 2), Very Slightly Included (VS 1 and VS 2), Slightly Included (SI 1 and SI 2), and Imperfect Included (I1, I2, and I3). Though flawless stones exist, they're incredibly rare -- most jewelers have never even seen one. As a result, many jewelry stores sell stones in the VS to SI range, which are considered "eye clean." The inclusions in this range are visible only under 10x magnification, says Moses. And though it may seem counterintuitive, a microscopic inclusion or two can actually be a good thing. "Like fingerprints, they make your stone unique and can help identify it," he says. Stones in the I range, however, should generally be avoided.