It's important to define some concepts in order to learn about the clarity of these precious gems. First of all, let's classify the imperfections that can affect a diamond. We can divide those in two categories, inner (inside the gem) and outer (in the surface).

Inner imperfections (inclusions)
These ones are from natural source. During the formation stage, microscopic substances called inclusions are formed inside the gems, those are usually from mineral origin (carbons), cracks in the inner side of the diamond (feathers), white spots (crystals), or very little points, smaller than the crystal ones (pinpoints).

Outer imperfections (blemishes)
Are caused by the jeweler, principally in the cut and polishing stages of the diamond refinement process, and we call them blemishes. These can be simple cracks in the diamond surface (fractures), groups of fractures located in an edge of the gemstone (bearding), additional polished facets, that break the symmetry of the diamond (extra facets), lack of polished parts (natural), or extra lines that were made in the polishing process (polishing lines), mistakes in the cut stage (chips), or abrasions (scratches).

Obviously, the most inclusions and blemishes the stone has, the lower will be its quality, and in consequence, the lower its value (price).

According to the IGI (International Gemological Institute), the different diamond clarity levels can be measured with a scale. On it, we can find from the 'perfect' diamonds, labeled as Flawless, to the most affected by the inclusions (I series).

FL (Flawless) No intern or extern defects at all with a x10 augments magnifier
IF (Internally Flawless) No inclusions with a x10 augments magnifier bigger than 5u
VVS1/2 (Very Very Small Inclusions) Minor inclusions that can barely be seen with a 10x magnifier
VS1/2 (Very Small Inclusions) Little inclusions barely visibles with a 10x magnifier
SI1/2/3 (Small inclusions) Little inclusions easily spotted with a 10x magnifier, but invisible naked eyed
I1 (Included) Inclusions easily spotted with the 10x magnifier and hardly seen with naked eye, not affecting the brilliance
I2 (Included) Easily spotted inclusions, that affect softly the brilliance
I3 (Included) Easily spotted inclusions that sightly affect the brilliance

The F and IF grades are very rare and expensive, only for the high budgeted ones, VVS1/2 grades are usually expensive and uncommon, VS1/2 are still very good quality but lower on price, so this grades are interesting for the 'standard budget'. SI1 have still a great quality with affordable prices, but SI2/3 (the 3 is not really recognized grade by the IGI, but most jewelers do, that is because we have included it here) start to be borderline quality. All the I series are low quality and we do not recommend purchasing one of those, unless the budget is really low. (A diamond is a diamond!)

The recommended quality grades for the standard budgets are (from most to less expensive) VS1 - VS2 - SI1, being this last one maybe the most attractive for the common customer, since it's possible to manage a purchase of this quality level for the price of the I series.

Usually the grades SI2, I1, I2 and I3 can be spotted with naked eye

Even if we are not using the magnifier, it's good to know about this scale and keep the advices in mind, so we will refine the final decision that fits the most for our needing and budget, and over all, we will not get scammed! *