Clarity refers to the presence (or absence) of impurities, blemishes or other identifying characteristics within a diamond. Clarity characteristics are what make every diamond completely unique. There are no two diamonds that will have exactly the same inclusions in the same location. There are five factors that determine how a major laboratory like GIA or AGSL will assign a clarity grade to a specific diamond.
Size : How large or small is an inclusion within a diamond?
Number : How many inclusions are found within a diamond?
Position : What is the location of the diamond's inclusions? Inclusions located under the table or "heart" of the diamond affect clarity grade more so than inclusions located under the side facets.
Nature : Do the inclusions affect the durability of a diamond? Large feathers can be hazardous depending on their exact size and position within the diamond.
Relief : How visible is the inclusion? Dark inclusions are easier to locate than a white or clear inclusion and may be graded more harshly.