The blue diamond is one of the most celebrated of all fancy colored diamonds. These gemstones are extraordinarily rare, representing only a tiny fraction of all diamonds unearthed globally. Their scarcity and striking color, which ranges from a whisper of sky blue to a dramatic, deep ocean shade, make them highly desirable. This combination of natural beauty and geological rarity has cemented the blue diamond’s reputation as a symbol of luxury and prestige.
The Spectrum of Blue
The appearance of a blue diamond is a wide spectrum of hue, tone, and saturation, which determines its visual character. Gemological laboratories use a specific scale to grade this color intensity, starting with Faint Blue and moving through Very Light Blue and Light Blue. These lighter grades possess a subtle, delicate tint that is noticeable but not overpowering.
The most sought-after categories fall under the “Fancy” designation, which includes Fancy Light Blue, Fancy Blue, Fancy Intense Blue, Fancy Deep Blue, and the rare Fancy Vivid Blue. Fancy Vivid Blue stones display the highest degree of saturation and a pure color that appears bright and rich. The most valuable diamonds are those that show a pure blue without any modifying colors.
Most blue diamonds display a secondary color, often described as a modifier. The most common secondary hues are grayish-blue or greenish-blue, which result from slight variations in their chemical composition. A diamond might be graded as “Fancy Grayish Blue” or “Fancy Greenish Blue,” where the modifying color is mentioned first to indicate its contribution to the overall look.
The Cause of the Color
The blue color in these diamonds is due to the presence of a trace element called boron within the crystal structure. Blue diamonds are classified as Type IIb, meaning they contain either very low or no nitrogen impurities.
During the diamond’s formation deep within the Earth, a boron atom replaces a carbon atom in the diamond’s crystalline lattice. This substitution creates a structural defect that selectively absorbs light in the red and yellow parts of the visible spectrum. The remaining light transmitted through the stone appears blue.
The concentration of boron atoms directly influences the depth and saturation of the final color. A faint blue diamond contains only minute traces of boron, whereas a stone graded Fancy Deep Blue or Fancy Vivid Blue holds a higher concentration of the element. This mechanism is the sole reason for the natural blue appearance.
Rarity and Major Sources
Blue diamonds are exceptionally rare due to the conditions required for their formation. These stones originate much deeper within the Earth than most other diamonds, forming at depths of 410 to 660 kilometers or more—up to four times deeper than colorless diamonds. The boron that creates the color is believed to have originated from the ancient seafloor, carried into the deep mantle through tectonic plate subduction.
Historically, the Golconda region of India produced some of the earliest and most famous blue diamonds, including the Hope Diamond, though that source is no longer active. Today, the most significant source for these rare gems is the Cullinan Mine in South Africa, which has yielded many of the world’s large and notable blue diamonds. The Argyle Mine in Australia also produced blue diamonds before its closure.
The extreme rarity means that blue diamonds constitute only a minute fraction of the total diamond production worldwide. Their Type IIb nature and deep-earth origin make finding a sizable, highly saturated blue diamond an event of major importance in the gemological world.
Evaluating Quality and Value
When evaluating a blue diamond, the 4 Cs (Cut, Clarity, Carat Weight, and Color) are applied, but their order of importance is altered. For fancy colored diamonds, the Color grade is the paramount factor, outweighing the influence of the other three characteristics. The hue, tone, and saturation determine the vast majority of the stone’s value.
The intensity grade translates directly into the diamond’s market value. A jump in color intensity, such as from Fancy Light Blue to Fancy Intense Blue, can result in an exponential increase in price. Stones with a pure blue hue, especially in the Fancy Vivid grade, command the highest prices.
Clarity and Cut, while still assessed, are less impactful on the overall value than they would be for a colorless diamond. In a deeply saturated blue diamond, small inclusions are often masked by the intense color, making high clarity less critical. Gemological Institute of America (GIA) reports are used to definitively grade these stones, ensuring the color’s natural origin and its precise intensity level.