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 and 62 percent, other proportions are important too. Remember that GIA doesn’t consider individual proportions in isolation.
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Diamond Color, Clarity, Cut and Carat Weight are collectively termed the 4Cs – the factors that, when combined, define a diamond’s quality and ultimately determine its value. GIA created the 4Cs of Diamond Quality, which has become the universal method for assessing the quality of any diamond, anywhere in the world. The creation of the 4Cs means that diamond quality can be communicated in a universal language so diamond purchasers know exactly what they are buying.
4C's INTERACTIVE TOOL
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GIA’s diamond color grading system measures the absence of color, starting with D as colorless and continuing to Z representing light yellow or brown. The distinctions between color grades are so subtle that they are often invisible to the untrained eye but can make a big difference in diamond quality and price.
A diamond’s cut determines its sparkle. To fashion a stone with proportions, symmetry and polish worthy of an excellent cut grade requires artistry and workmanship. The finer the cut quality, the more sparkle the diamond has.
Natural diamonds form from carbon exposed to tremendous heat and pressure deep in the earth. Diamonds often contain clarity characteristics, called inclusions or blemishes. Inclusions are enclosed within the gem and blemishes are on the surface of the diamond. If all else is equal, the closer a diamond is to flawless, with no inclusions or blemishes, the higher its value.
Diamond weight is measured in carats. One carat is equal to 0.2 gram, about as heavy as a paperclip. Since larger diamonds are more rare, they will cost more than a smaller gem with the same color, clarity and cut grades.
The anatomy of a diamond has 9 parts:
The diamond’s overall depth from the surface of the table to the culet, expressed as a percentage of the average girdle diameter.
The pavilion is the lower portion 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 is another important dimension of the stone, 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 and 41.8 degrees to be considered “Excellent,” providing other parameters also fit their proper ranges.
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.
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 route 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
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.
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.
GIA GRADING REPORTS
A diamond grading report is the scientific blueprint of a stone’s quality characteristics. A GIA diamond grading report is your assurance that your diamond is a natural diamond, with disclosure of any treatment to enhance color or clarity. The report provides the clear evidence that is vital to a confident purchase. A GIA Diamond Grading Report not only provides expert analysis of Color, Cut, Clarity and Carat Weight, it also contains a plotting diagram that clearly shows a diamond’s inclusions and clarity characteristics. The GIA Diamond Dossier® includes these without the graphical representation of the clarity characteristics. All GIA reports contain security features to prevent them from being forged or duplicated. GIA does not buy or sell diamonds, making an independent diamond grading report from GIA an unbiased assessment of the diamond.