Average Girdle Diameter
The result of adding the smallest and largest girdle measurements of a round brilliant and dividing by two.
Belly
The slightly curved center of the long side of the pear, marquise, heart, or oval.
Blemish
A clarity characteristic confined to the surface of a polished diamond.
Bow-tie
The dark area across the center of an elongated brilliant cut.
Brightness
The effect of all the diamond's internal and external reflections of white light.
Brilliance
The brightness that stems from the center of the diamond. Brilliance occurs when light enters through the table, reaches the pavilion facets, and is reflected back through the table.
Brilliant Cut
A cutting style in which triangular and kite shaped facets spread out from the diamond's center toward the girdle.
Carat (aka ct.)
The unit of weight in which a diamond is measured.
Carbon Spots
An included crystal that has a dark appearance instead of a white or transparent appearance. In many cases dark inclusions are not visible to the unaided eye and do not affect the brilliance.
Clarity
Absence of internal inclusions and external blemishes. The GIA clarity grading scale is FL (flawless), IF (internally flawless), VVS1 and VVS2 (very, very, slightly included), VS1 and VS2 (very slightly included), SI1 and SI2 (slightly included), and I1, I2, I3 (imperfect).
Clarity Characteristics
Internal or external feature of a diamond that helps determine the quality and establish its identity.
Color Grading
A system of grading diamonds based on either being colorless for white diamonds or hue, depth of color, and saturation of color for fancy color diamonds. For white diamonds, GIA uses a scale of D-Z in which D is colorless and Z is light yellow. Z should not be confused with yellow fancy color diamonds.
Crown
The upper portion of the diamond above the girdle. The crown contains a table facet and depending on diamond shape either star and bezel facets (round brilliant, oval, pear, radiant, princess, heart) or rows of concentric facets reaching from the table to the girdle (emerald, asscher).
Crown Angle
The angle formed by the bezel facets and the girdle plane.
Crown Height Percentage
The distance from the girdle plane to the table expressed as a percentage of average girdle diameter.
Culet
A facet sometimes added to the bottom of the pavilion to protect the tip.
Cut
The proportions and finish of a polished diamond. Many Gemologists believe cut is the most important of the 4 Cs.
Cutting Style
The arrangement of a diamond's facets.
Depth
The height of a diamond from the culet to the table and is measured in millimeters.
Depth Percentage
A ratio of the total depth of a diamond from table to culet compared to the total diameter.
Diamond
A crystal made up of 99.5% pure carbon atoms arranged in an isometric, or cubic, crystal arrangement.
Dispersion
When white light passes through the diamond and is broken up into spectral colors.
Eye Clean
A term used to describe a diamond with no blemishes or inclusions that can be seen with the unaided eye.
Face-up (aka table up)
A position where the diamond's crown and table are pointed towards the viewer.
Face-down (aka table down)
A position where the diamond's pavilion is pointed towards the viewer.
Facet
A smooth flat plane on the surface of a diamond. Facets allow light to enter a diamond and reflect off its surface at different angles creating color and light.
Fancy Cut
Any diamond shape other than a round.
Feather
A term for any break in a diamond.
Finish
A term used to describe the quality of the polish and symmetry of the cut in a fashioned diamond.
Fire
The flashes of colors in a polished diamond.
Fluorescence
The emission of visible light by a diamond when it is exposed to ultraviolet radiation.
French Tips
A cutting style that replaces the large bezel facets at the points of a marquise, pear, and heart with star and upper girdle facets.
Girdle
The outer edge or outline of the diamond's shape.
Girdle Thickness Percentage
Girdle thickness expressed as a percentage of the average girdle diameter.
Inclusion
A clarity characteristic totally enclosed in a polished diamond or extending into it from the surface.
Length to Width Ratio
A numerical expression between the length of a diamond and the width of a diamond where the value for width is one.
Lower Girdle Facet Percentage
The length of the lower girdle facets expressed as a percentage of the total distance between the girdle and the culet.
Luster
The appearance of the diamond's surface in reflected light.
Mixed Cut
A cutting style that combines brilliant cut facets and step cut facets.
Natural
Small parts of the original rough diamond's surface which are left on a polished diamond.
Normal Color Range (aka the D-Z range)
The color range of white diamonds from colorless to light yellow or light brown.
Pavilion
The lower portion of a diamond below the girdle.
Pavilion Angle
The angle formed by the pavilion main facets and the girdle plane.
Pavilion Bulge
Outward curve of the pavilion facets on a step cut diamond.
Pavilion Depth Percentage
The distance from the girdle plane to the culet expressed as a percentage of average girdle diameter.
Plot (aka diamond certificate)
A map of a diamond's inclusions, blemishes, and facet arrangements.
Point
A unit of measurement used to describe the weight of a diamond.
Polish
The overall condition of the facet surfaces of a finished diamond.
Proportions
The angles and relative measurements of a polished diamond and the relationship between them.
Reflection
The bouncing back of light when it strikes a surface.
Refraction
Change in speed and direction of light as it travels through a diamond.
Scintillation
The flashes of light and dark areas you see when the diamond, light, or observer moves.
Shape
The face-up outline of a diamond.
Shape Appeal (aka "eye-pleasing")
A diamonds overall appearance in relation to others of the same size, shape, and cutting style.
Sparkle
The spots of light that flash as the diamond, light, or observer moves.
Star Facet Length Percentage
The length of the star facets expressed as a percentage of the total distance between the girdle and the edge of the table.
Step Cut
A cutting style with long narrow four sided facets in rows parallel to the girdle on the crown and pavilion.
Symmetry
Refers to the exactness of a finished diamond's shape and the placement of the facets.
Table
The largest facet on the diamond.
Table Percentage
The diamond's table size expressed as a percentage of its average girdle diameter.
Total Depth Percentage
Table to culet depth expressed as a percentage of the average girdle diameter. 
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