Verena Pagel-Theisen, Diamond Grading ABC: Handbook for Diamond Grading, 1980, p.154
Parts that make up a cut diamond
Table: The flat facet on the top of the diamond. It is the largest facet on a cut diamond.
Crown: The upper part of the diamond above the girdle. Consists of a large flat area on top called a table, and several facets below it
Girdle: The outer edge or the widest part of the diamond forming a band around the stone.
Pavilion: The bottom part of the Diamond, below the girdle.
Culet: A tiny flat facet that diamond cutters sometimes add at the bottom of a diamond’s pavilion. Its purpose is to protect the tip of the pavilion from being chipped or damaged. Once a diamond is set in jewelry, though, the setting itself generally provides the pavilion with sufficient protection from impact or wear. Large or extremely large culets were common in diamonds cut in the early part of this century, such as the Old European or Old Mine Cut. However, such large culets are rarely seen today. Most modern shapes have either no culet at all, or a small or very small culet.
Depth: The height of a diamond from the culet to the table. The depth is measured in millimeters.
Crown angle: The angle at which a diamond’s bezel facets intersect the girdle plane. This gentle slope of the facets that surround the table is what helps to create the dispersion, or fire, in a diamond. White light entering at the different angles in broken up into its spectral hues, creating a beautiful play of color inside the diamond. The crown angle also helps to enhance the brilliance of a diamond.
Table percentage: The value which represents how the diameter of the table facet compares to the diameter of the entire diamond. So, a diamond with a 60% table has a table which is 60% as wide as the diamond’s outline. For a round diamond, gemologists calculate table percentage by dividing the diameter of the table, which is measured in millimeters (this millimeter measurement does not appear on diamond grading reports) by the average girdle diameter. For a fancy shape diamond, table percentage is calculated by dividing the width of the table, at the widest part of the diamond, by the millimeter width of the entire stone.
Facet: The smooth, flat faces on the surface of a diamond. They allow light to both enter a diamond and reflect off its surface at different angles, creating the wonderful play of color and light for which diamonds are famous. The table below shows all the facets on a round brilliant cut diamond. A round brilliant has 58 facets (or 57 if there is no culet).
Symmetry: Refers to variations in a diamond’s symmetry. The small variations can include misalignment of facets or facets that fail to point correctly to the girdle. Symmetry is regarded as an indicator of the quality of as diamond’s cut; it is graded as either Ideal, Excellent, Very Good, Good, Fair or Poor.
Fluorescence: An effect that is seen in some gem-quality diamonds when they are exposed to long-wave ultraviolet light (such as the lighting frequently seen in night clubs). Under most lighting conditions, this fluorescence is not detectable to the eye. However, if a diamond is naturally fluorescent, it will emit a soft colored glow when held under an ultraviolet lamp or “black light.” Fluorescence is not dangerous to the diamond or to the wearer; it is a unique and fascinating quality that occurs naturally in a number of gems and minerals. Most commonly diamonds fluoresce blue, but can also fluoresce yellow or white depending on which trace elements are found in the matrix of the diamond crystal. Fluorescence is described as none (or inert), faint, negligible, medium, strong and very strong.
Fluorescence in diamonds can have both favorable and undesirable effects. For example, diamonds in the color range of I to N with medium to strong blue fluorescence can appear more colorless, which is an obvious advantage. The blue acts to mask or offset the very faint body color of these diamonds. On the other hand, diamonds with very strong fluorescence can exhibit an oily or milky appearance, even under incandescent lighting. However, according to a GIA study, the average observer could not detect any difference in color or transparency when viewing diamonds with fluorescence.