Why the world's most valuable diamonds are graded starting with the letter D

Countries & Cities · Published 15 October 2026 · Updated 25 October 2026 Swipe more like this →

In short: Most diamonds are valued for their lack of color, yet the industry uses a grading scale that begins at D rather than A. This deliberate choice was designed to eliminate confusion with inconsistent, older systems and establish a universal standard for assessing the subtle hues found in natural stones.

In the diamond trade, color is a primary factor of value, yet it is often defined by the absence of hue. Chemically pure, structurally perfect diamonds are transparent. However, most natural diamonds contain impurities or lattice defects that introduce tints. While the normal color range spans from colorless to pale yellow or brown, fancy diamonds—such as rare pinks, blues, or reds—are valued for their intense saturation. These fancy colors arise from specific conditions, such as plastic deformation of the crystal lattice or the presence of elements like boron.

Historically, diamond grading lacked consistency, with various systems using letters, Roman numerals, or descriptive terms like 'river' or 'cape'. To bring order to the market, the Gemological Institute of America (GIA) introduced the D-to-Z scale. The choice to start at D was intentional; by skipping A, B, and C, the GIA ensured that its new, standardized grades would not be confused with the subjective, legacy terminology that previously caused misrepresentation and misunderstanding in the trade.

Grading requires expertise and precise conditions. Professionals compare an unset diamond against a set of master stones—samples representing the minimum color for each grade—inside a controlled lighting box. While the D-to-Z scale is the global standard for the normal color range, fancy-colored diamonds are evaluated using a different, more complex system that accounts for hue, tone, and saturation. Ultimately, rarity drives value: colorless stones are prized for their scarcity, while vivid fancy colors command record-breaking prices at auction, such as the 14.62-carat Oppenheimer Blue, which sold for $50.6 million in 2016.

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