Before reading about the cut of a diamond, you should be familiar with a few general terms.

Facet:     A smooth, flat face on the surface of a diamond.

Table:     The flat facet on the top of a diamond.

Girdle:     The narrow band around the widest part of a diamond.

Crown:     The top portion of a diamond extending from the girdle to the table.

Culet:     The tiny, flat facet at the bottom of a diamond.

Pavilion:     The bottom portion of a diamond extending from the girdle to the culet.

Diameter:     The width of a diamond measured through the girdle.

Depth:     The height of a diamond from the culet to the table.

Cut

When gemologists talk about diamond cut, they are referring not to its shape, but to its proportions - its depth, width, and the uniformity of its facets. These characteristics control how light travels within a diamond and gives it its brilliance.

Diamond Depth and Width

The proportions of depth and width have the greatest impact on diamond brilliance, the reflection of white light that we see when we look at a diamond.

As shown in the image below, if cut is too shallow, light traveling through the diamond is lost out the bottom. This loss of light makes shallow cut diamonds appear dull and lifeless.

Conversely, if cut is too deep, the light traveling through the diamond escapes out the sides, causing the diamond to appear dark.

In an ideal cut diamond light bounces back out the top of the stone bringing its brilliance into view.

Diamond Polish and Symmetry

Polish refers to the smoothness of a diamond's facets. With poor polish, the surface of a facet will create a dulled sparkle. A smooth polish allows light to easily pass through the surface of a facet.

Symmetry refers to the alignment of a diamond's facets. With poor symmetry, light doesn't reflect as it should because the diamond's facets are misaligned.

Cut Grading

The cut grade reflects the combined impact of a diamond's proportions, symmetry, and polish - all three of which impact the diamond's beauty and value. 

Gemological Institute of America (GIA) reports do not grade diamonds cuts, however, the American Gem Society Laboratories (AGSL) reports do.

The AGSL use a cut grading system that assigns diamonds a cut grade from a range of 0 to 10.

"0" or "Ideal Cut"

The AGSL Ideal "0" Cut rating is given to diamonds that have the very best cutting. In these diamonds the cut has correct proportions, precise symmetry and fine polish. All three combine to create the best possible display of brilliance, scintillation, and dispersion.

Other Grades

As diamonds fall into grades further from 0, their relative beauty begins to diminish. For example, a diamond with a cut grade 1 is very close to 0 in appearance. A cut grade of 2 is still considered to have very fine proportions and deliver exceptional brilliance.

Symmetry Grading

GIA, AGSL as well as EGL-USA labs provide the following symmetry and polish grades. The table describes the terminology used by each lab: