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Diamond Grading

NORTH AMERICAN GEMOLOGICAL LABORATORIES, LLC (NA-GL) are experts in diamond grading. Diamond grading determines the dollar value. It is based on the “4 C’s”: color, clarity and cut (proportion), which are the quality elements, together with the carat weight determine value of a stone. Each element is based on a scale. The closer a diamond grades to the left of one or all of the scales, the rarer and more valuable the stone. Clarity is frequently assumed the most important. However, color and cut, especially cut, have the greatest affect on the appearance of a diamond.

The 4 C's

Cut

The cut of a diamond greatly determines its brilliance. A cut grade is based on 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. High quality diamonds are cut to strict proportions. As a result, they are extremely bright and beautiful. If the cut on a high quality diamond is fair or poor than the diamond lacks brilliance.

GIA cut scale is five grades: Poor, Fair, Good, Very Good, and Excellent.

Determining a diamond's cut grade, goes beyond simple measurements of width and depth. Using a proportion analyzing device, a three-dimensional model is created to determine the diamond's proportions and angles. The relationships between the various dimensions greatly affect light’s reaction once it enters and exits the diamond. Sophisticated computer software and hardware allows light tracing to measure the levels of brightness, fire and scintillation.

Polish and symmetry are two important aspects of the cutting process. Polish grade describes the quality of the diamond's facet surface and symmetry grade refers to facet alignment. Diamonds with a poor polish are dull. Diamonds with poor symmetry leak light, which results in light rays escaping through the pavilion (lower half of the stone). Light is not reflected back to the viewer’s eye, limiting a diamond’s sparkle.

To grade the cut of a diamond, NA-GL evaluates the following:
  • Overall Shape or Outline
  • Diameter
  • Total Depth Percentage
  • Crown Angle
  • Crown Height
  • Pavilion Angle
  • Pavilion Depth
  • Culet Size
  • Girdle Thickness
  • Table Size
  • Total Depth
  • Polish
  • Symmetry
  • Brilliance
  • Fire
  • Scintillation

Color

Grading color in the normal range involves determining how close a stone’s color approaches colorlessness. Most diamonds have a trace of yellow or brown body color, with exception of some natural fancy colors, such as blue, pink or red. The colorless grade is the most valuable. Fluorescence (define) can enhance the perceived color.

Clarity

Clarity refers to a stones relative position on a flawless-to-perfect scale. Clarity characteristics are classified as inclusions (internal) or blemished (external). The size, quantity, position, nature, color and relief of the characteristics determine the clarity grade. Very few diamonds are flawless, showing no inclusions or blemishes when examined by a skilled grader under 10X magnification. If other factors are equal, flawless stones are the most valuable.

Carat Weight

The final aspect of grading a diamond is the carat. Carat is a metric unit of weight in the gem stone industry. Carat is equal to 1/5th of a gram or “5 carats in a 1 gram.” The term “points” is a decimal fraction of a carat. A point is equal to .01 (1/100th) of a carat. Just as 1 dollar is equal to 100 cents, 1 carat equals 100 points.

Generally speaking, larger stones are rarer than smaller stones of the same quality. This is why the 1 carat, 20 stone cluster ring is less expensive than a 1 carat solitaire of the same quality. The size at which a gemstone increases significantly in price varies with the availability.

Carat weight and the physical size of a stone vary between species of gemstones. For example, a carat round diamond is 6.5 millimeters in diameter, but a 1 carat ruby is 5.5 to 6.0 millimeters in diameter. This is because rubies are denser and typically cut heavier than a diamond. So, a 1.5 carat stone may be needed to replace the mounting of a 1 carat diamond. Physical measurements rather than carat weight should be used to match size.