Diameter: Width of the diamond at the widest point of the girdle (in fancy cut diamonds, the smallest diameter is used)
Table: The largest facet at the top
Crown: The top part of the diamond between the girdle and the table
Girdle: The narrow band at the diamond’s widest point
Pavilion: The bottom part of the diamond between the girdle and the culet
Culet: The fact at the bottom tip (preferable when not visible to the naked eye)
Depth: >The height of a diamond from the table to the culet
Facet: A surface or ‘face’ created when the diamond is cut
Star Facet: Triangular shaped facets surrounding the table which form a star-shape when viewed from above
Upper Girdle Facets: The 16 triangular facets that sit directly on top of the girdle
Lower Girdle Facets: The 16 triangular facets that sit directly below the girdle
Pavilion Main Facets: The 8 large four-sided facets which run from the girdle to the culet
Table Size: The table is the top horizontal facet of the diamond. The average table size is expressed as a percentage of the diamond’s average girdle diameter. While an “Excellent” grade diamond will have a table size between 52 to 62 percent, other proportions are important too. Remember that GIA doesn’t consider individual proportions in isolation.
Total Depth: The diamond’s overall depth from the surface of the table to the culet, expressed as a percentage of the average girdle diameter.
Pavilion Depth: The pavilion is the lower proportion of a diamond from the bottom edge of the girdle to the culet. The pavilion depth is expressed as a percentage of the average girdle diameter. A pavilion depth that’s too shallow or too deep will allow light to escape from the side or bottom of the stone. A well-cut diamond will direct more light upward through the crown.
Pavilion Angle: Pavilion angle is another important dimension of the diamond, especially as it relates to a diamond’s brightness. It is the average of the angles formed by the diamond’s pavilion main facets and its girdle plane. This should fall between 40.6 to 41.8 degrees to be considered “Excellent”, providing other parameters also fit their proper ranges.
Crown Height: The crown is the upper portion of the diamond, from the top edge of the girdle to the table. The average crown height is expressed as a percentage of the average girdle diameter. It can affect both the dispersion and brightness of a diamond.
Crown Angle: The crown angle is the angle that’s formed where the bezel facets meet the girdle plane. The crown angle in a well-cut diamond will be within 31.5 to 36.5 degrees. The crown angle has a large effect on the face-up appearance of a round brilliant-cut diamond. The best range of crown angles provide a routh for exiting light dispersion, as well as additional contrasting directions for entering light. Star Length: The horizontally projected distance from the point of the star facet to the edge of the table, relative to the distance between the table edge and the girdle edge. A well-cut diamond will range between 40 to 70 percent, when other parameters are within the correct ranges
Girdle thickness: The girdle is the middle portion of a diamond, a narrow section separating the crown from the pavilion, and functions as the diamond’s setting edge. The girdle thickness is described as a range from its thinnest to thickest areas. A thick girdle is less desirable because it unnecessarily adds weight to the stone where it matters the least (making the diamond appear smaller). An extremely thin girdle is sometimes referred to as a knife-edge, and results in a diamond that is more fragile and susceptible to chipping. Therefore, a girdle that is “medium to slightly thick” is preferred.
Lower Girdle/ Half Facet Length: This ratio is measured by calculating how long the lower girdle facets are relative to the length of the pavilion. This defines the contrast of a round brilliant cut diamond, which controls the brilliance of a diamond. The well-cut diamond will range between 65 to 90 percent. Diamonds with longer lower half facets will have a little more scintillation.
Culet: The culet is the small facet at the bottom of a diamond intended to prevent chipping and abrasion to the point. The culet size can affect face-up appearance and it’s described as the average width of the facet. Size is expressed as none, very small, small, medium, slightly large, large, very large, and extremely large. When there is no culet, it is sometimes referred to as a pointed culet. Preferably the culet will not be visible with the unaided eye, and when described as none (no culet), very small or small on a GIA report, it falls in the excellent range.