The graphic below explains the major parts of a diamond. When considering diamond anatomy, the three most important components are diameter, table, and depth. This is because the ratio of the table to the diameter, and the depth to diameter, figure prominently into determining a diamond's cut grade.
While understanding a diamond's anatomy can be helpful, it should not supersede the importance of or be confused with cut grade. The various parts of a diamond and how well they are cut are included as part of a diamond's cut grade.
Diameter: The width of the diamond as measured through the girdle.
Table: The largest facet of a gemstone.
Crown: The top portion of a diamond extending from the girdle to the table.
Girdle: The intersection of the crown and pavilion which defines the perimeter of the diamond. While generally a minor consideration, Blue Nile recommends avoiding girdles graded either extremely thin, which makes diamonds more susceptible to chipping, or extremely thick, which puts too much weight in the middle of the diamond, causing it to look smaller than diamonds of similar weight.
Pavilion: The bottom portion of a diamond, extending from the girdle to the culet.
Culet: The facet at the tip of a gemstone. The preferred culet is not visible with the unaided eye (graded "none" or "small").
Depth: The height of a gemstone measured from the culet to the table.