A diamond is a crystal lattice composed entirely of carbon atoms, formed under immense heat and pressure deep within the Earth’s mantle. In its purest form, a diamond is colorless and transparent because its structure does not interact with visible light. Color variations occur when foreign atoms or structural defects are incorporated into the carbon framework during formation. Yellow is the most common natural color found in diamonds, stemming from a specific atomic impurity that alters the stone’s optical properties.
The Role of Nitrogen Impurities
The presence of nitrogen atoms is the cause of yellow coloration in diamonds. Nitrogen is the most common impurity found in natural diamonds, as its atomic size is similar enough to carbon to be incorporated into the crystal structure. During the diamond’s growth process, nitrogen atoms substitute for carbon atoms within the lattice, introducing a structural irregularity.
These nitrogen impurities selectively absorb light in the blue-to-violet portion of the visible spectrum. When white light enters the diamond, the blue component is absorbed by the nitrogen centers. The light transmitted back to the observer is missing the blue wavelengths, causing the remaining combination of colors to appear yellow.
The overall concentration and arrangement of these nitrogen atoms determine the diamond’s classification. Diamonds are categorized into Type I (containing nitrogen) and Type II (virtually no nitrogen). Type I diamonds are further divided into Type Ia, where nitrogen atoms are clustered, and the much rarer Type Ib, where nitrogen atoms are dispersed as isolated single units. Type Ib diamonds tend to exhibit a more intense yellow hue compared to Type Ia diamonds.
Nitrogen States and Color Intensity
How nitrogen atoms cluster together dictates the shade and saturation of the yellow color. Most natural diamonds are Type Ia, meaning their nitrogen exists in aggregated forms, which often results in a faint to light yellow color. The simplest clusters are A-aggregates (two nitrogen atoms bonded together) and B-aggregates (four nitrogen atoms clustered around a vacant lattice site).
These A- and B-aggregates typically produce a pale yellow tint, often referred to as the “Cape series” color. This pale color results from the nitrogen having aggregated over long periods at high temperatures.
The rarest and most desirable intense yellow colors, known commercially as Fancy Yellow, are caused by two primary atomic configurations. One configuration is the isolated nitrogen atom, or C-center, found in Type Ib diamonds, which strongly absorbs blue light to create a deep yellow.
The other primary cause of intense yellow is the N3 center. This complex defect consists of three nitrogen atoms grouped next to a single vacant position in the carbon lattice. The N3 center is the most common color-causing defect in natural fancy yellow diamonds, creating a vivid, saturated color.
The Diamond Color Grading Scale
The commercial grading of yellow diamonds uses two distinct scales based on color intensity. The standard D-to-Z scale is used for colorless to near-colorless diamonds. On this scale, D represents a completely colorless stone, and grades progress as the diamond shows an increasing presence of yellow or brown tint.
A diamond graded Z has the deepest concentration of pale yellow color within this near-colorless range. The Z grade is the point where the nitrogen-caused coloration becomes clearly visible, but it is not yet considered a “Fancy” color. Diamonds exhibiting a color stronger than Z move to the second system, the Fancy Color Grading Scale.
The Fancy Color scale describes the saturation and tone of the yellow color. Categories start with Fancy Light Yellow, progressing to Fancy Yellow, Fancy Intense Yellow, and the most saturated, Fancy Vivid Yellow or Fancy Deep Yellow. Unlike the D-Z scale where color lowers value, the value of a Fancy Yellow diamond increases with the intensity and purity of the yellow hue.
Modifying Yellow Color Through Treatment
The natural yellow color of a diamond can be intentionally altered by laboratory processes to change its appearance. One common method is High Pressure/High Temperature (HPHT) treatment, which mimics the natural conditions deep inside the Earth. HPHT processing can be used to either remove or intensify yellow color, depending on the starting material.
HPHT treatment can be applied to off-color brown or yellowish diamonds to encourage the aggregation of nitrogen defects, thereby removing the color and resulting in a near-colorless stone. Conversely, HPHT can also convert certain diamonds with brown coloration into a stable, attractive yellow hue.
Another technique involves irradiation, which introduces structural defects that cause a color center, often followed by a low-heat process called annealing. This combination can create or deepen yellow and orange hues. These processes are stable and permanent, but they must always be disclosed to distinguish the artificially enhanced color from the naturally occurring shade.