Clarity refers to the presence of inclusions and blemishes in a diamond. Since a diamond is a natural substance, gemologists refer to these inclusions or patterns of inclusions as "identifying characteristics," and consider them to be a diamond's unique "fingerprint."
Inclusions could help in diamond's identification. Examples of the types of internal inclusions are: pinpoint - an included crystal that is transparent, opaque or dark clouds: groups of pinpoints feathers: fractures
External blemishes include polishing lines, grain lines (crystal growth twinning planes), scratches, chips, nicks and "naturals" (part of the rough diamonds original surface). Diamonds are graded for clarity according to the number, size, location and type of inclusion.
Diamonds are free from internal and external imperfections when examined by skilled professionals under natural or artificial light with a 10X loupe corrected for chromatic and spherical aberration or with a 10X binocular microscope equipped with dark field illumination.
The following conditions still qualify a diamond as flawless:
Though a diamond is said to be "flawless," if no inclusions (internal or external) can be seen under 10x magnification (by a triplet loupe or a binocular microscope), truly flawless diamonds are extremely rare.
This means that no inclusions visible under 10x magnification. Diamonds in this category are free from all internal imperfections (inclusions) but do possess minor surface blemishes. These diamonds may be made flawless by minor repolishing with the exception of surface grain lines.
Inclusions that are very small but will be easy for an expert to find under 10x magnification. These grades imply minor inclusions of the size, number and location that stand between those somewhat difficult to observe and those somewhat easier to observe. Small included crystals, small feathers, distinct clouds characterize VS grades.
The SI grade category is the most open to interpretation, and the stones in this grade are very often subject to qualifiers such as “Good SI” and “Bad SI.” A “Good” SI could turn out to be spectacular, making it one of the best buys on the market.
Unfortunately, the GIA grading system does not address this disparity, leaving the field wide open to “creative salesmanship.”
It also contains noticeable inclusions which are easily visible under a 10x magnification. Usually these inclusions will be centrally located and noticed immediately when the diamond is examined. Diamonds in these grades (particularly the SI2 grade) may show inclusions to the unaided eye when placed table down on a white background, but not when viewed face-up.
Despite this definition, it is possible for a trained eye to see the inclusions when the stone is facing up - in SI2 in particular.
The "Imperfect Categories" contain obvious inclusions when viewed under a 10x loupe and are visible to the unaided eye in the face-up position. The grades vary from diamonds with inclusions that are difficult to locate by the unaided eye, to those with severe inclusions that are easily noticeable
Contain inclusions that might or might not disturb the pattern of reflections in a stone. I1 stones with translucent, non-reflective inclusion or inclusions located in a place where it could be covered by prongs are highly desirable.
Large inclusions, or many of them, that slightly diminish the brilliance of the stone to the naked eye.
Enough inclusions that the brilliance of the stone is noticeably diminished.