Most diamonds contain some inner flaws, or inclusions, that occur during the formation process.
The visibility, number and size of these inclusions determine what is called the clarity of a diamond.
Diamonds that are clear create more brilliance, and thus are more highly prized, and priced.
Because they are formed deep within the earth, under extreme heat and pressure; virtually all diamonds contain "birthmarks"; small imperfections inside the diamond (called inclusions), or on its surface (called blemishes). Clarity refers to the degree to which these imperfections are present. Diamonds which contain numerous or significant inclusions or blemishes have less brilliance because the flaws interfere with the path of light through the diamond.
The position of an inclusion affects how easily it can be seen. Diamond cutters make every effort to cut a stone so that inclusions are not visible through the table of the finished diamond. The preferred position for inclusions is under the bezel facets or near the girdle because they are harder to see there.
Almost all diamonds are graded for clarity using the 11 point diamond clarity scale created by the GIA. In grading diamond clarity, the GIA considers the number, size, colour, reflectivity, and position of every flaw visible under 10x magnification.