Masterstone & Color Grading Protocol

Color grading can feel subjective until the grader learns to standardize their environment exactly as the GIA dictates.

The Setup

A highly controlled environment is necessary to eliminate variables and isolate a diamond's true body color. The physical requirements for grading include:

• Lighting: Grading must be done under proper daylight-equivalent fluorescent lighting (typically color-balanced between 5500K and 6500K). The light should be UV-filtered to prevent fluorescence from interfering with the baseline color grade.
• Neutral Backgrounds: Diamonds must be evaluated against a completely neutral, non-fluorescent white background, such as a specialized white plastic grading trough or tray. Graders must also control for ambient color contamination (e.g., wearing neutral clothing, having neutral-colored room walls).
• The Viewing Angle: The subject diamond is never color graded face-up. It must be viewed table-down. The grader looks through the pavilion facets, with their line of sight parallel to the girdle. Looking down through the table introduces brilliance and dispersion (fire), which effectively mask the stone's body color.

Masterstone Technique

GIA color grading requires comparing the subject diamond to a highly precise, pre-graded set of D-Z masterstones. The physical motion and sequence are highly standardized:

• Preparation: Both the masterstones and the subject diamond must be thoroughly cleaned. A simple film of oil or dust can alter the perceived color grade.
• Placement: Place the subject diamond and the sequence of masterstones table-down in the V-groove of the white grading tray.
• The Physical Motion: The grader physically moves the subject diamond left and right along the tray, placing it adjacent to different masterstones to compare the depth of body color.
• Bracketing: The goal is to "bracket" the subject diamond. The grader compares the subject stone to a masterstone to determine if it has more color, less color, or the same amount of color. By moving the stone through the sequence, the grader finds the precise interval where the subject diamond is darker than the masterstone to its left, but lighter than the masterstone to its right.

Fluorescence

Fluorescence is the emission of visible light by a diamond when exposed to ultraviolet (UV) radiation. It is a vital characteristic that can significantly alter how a diamond looks in everyday lighting.

• Grading Levels: Evaluated under a long-wave UV light source (365 nm), the GIA categorizes fluorescence into five distinct levels of intensity:

  • None
  • Faint
  • Medium
  • Strong
  • Very Strong
• Influence on Perceived Color: Because the most common color of diamond fluorescence is blue, it can artificially influence a diamond's perceived color in natural sunlight (which contains UV). Blue is the complementary color to yellow. Therefore, Medium to Strong blue fluorescence can temporarily mask a diamond's yellowish body color, causing a lower-color diamond (e.g., J, K, L) to appear artificially whiter (higher in color grade) when worn outside.
• Visual Detriments: Conversely, in some rare cases, "Strong" or "Very Strong" fluorescence can cause a diamond to appear milky, hazy, or oily in daylight, negatively impacting its transparency and value.