Diamond Cut

The cut of a diamond determines its brilliance. There is no single measurement of a diamond that defines its cut, but rather a collection of measurements and observations that determine the relationship between a diamond's light performance, dimensions and finish. Most gemologists consider cut the most important diamond characteristic because even if a diamond has perfect color and clarity, a diamond with a poor cut will have dulled brilliance.

The width and depth can have an effect on how light travels within the diamond, and how it exits in the form of brilliance.

• Too Shallow:

  • Light is lost out the bottom causing the diamond to lose brilliance.

• Too Deep:

  • Light escapes out the sides causing the diamond to appear dark and dull.

Cut Determines Brilliance

Determining a diamond's cut grade, however, goes beyond simple measurements of width and depth. Using an optical measuring device, a three-dimensional model is created to determine the diamond's proportions and angles. The interrelations between these various dimensions will greatly affect how light reacts once it enters and how it behaves once it exits; by using sophisticated computer modeling, it is possible to trace light behavior and measure its levels of brightness, fire and scintillation - the face-up appearance.

Diamond Anatomy

• Diameter:

  • The width of the diamond as measured through the girdle.

• Table:

  • The largest facet of a gemstone.

• Crown:

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

• Girdle:

  • The intersection of the crown and pavilion which defines the perimeter of the diamond.

• Pavilion:

  • The bottom portion of a diamond, extending from the girdle to the culet.

• Culet:

  • The facet at the tip of a gemstone. The preferred culet is not visible with the unaided eye (graded "none" or "small").

• Depth:

  • The height of a gemstone measured from the culet to the table.

How GIA Grades Cut

GIA's cut grading system considers 7 qualities: Brightness, Fire, and Scintillation (which are considered the Face-Up Appearance components of the cut grade); Weight Ratio and Durability (the Design components); and Polish and Symmetry (the Craftsmanship components). GIA rates the Cut of diamonds as either Excellent, Very Good, Good, Fair, or Poor.

How AGS Grades Cut

AGS's cut grading system considers 11 qualities: Brightness, Dispersion, Leakage, and Contrast (the Light Performance components); Durability, Tilt, Weight Ratio, Girdle Thickness, and Culet Size (the Proportions components), and Polish and Symmetry (the Finish components). AGS rates the Cut of diamonds as either Ideal, Excellent, Very Good, Good, Fair, or Poor.

What Do These Cut Grades Mean?

1. Ideal cut:

   1. Represents roughly the top 3% of diamond quality based on cut. Reflects nearly all light that enters the diamond. An exquisite and rare cut.

2. Very good cut:

   1. Represents roughly the top 15% of diamond quality based on cut. Reflects nearly as much light as the ideal cut, but for a lower price.

3. Good cut:

   1. Represents roughly the top 25% of diamond quality based on cut. Reflects most light that enters. Much less expensive than a very good cut.

4. Fair cut:

   1. Represents roughly the top 35% of diamond quality based on cut. Still a quality diamond, but a fair cut will not be as brilliant as a good cut.

5. Poor cut:

   1. This includes all diamonds that do not meet the performance standards of a fair cut. These diamonds are generally deep and narrow or shallow and wide and tend to lose most of the light out the sides and bottom. Blue Nile does not carry diamonds with cut grades of poor.