Colored diamonds, often referred to as Fancy Color diamonds, represent an exceedingly rare segment of the gem market. While most mined diamonds are colorless, a distinct hue significantly elevates a diamond’s status and value. The striking colors originate through two fundamental pathways: billions of years of geological formation deep within the Earth or advanced post-mining treatment in a laboratory.
The Science Behind Natural Coloration
A diamond’s color is directly linked to the presence of trace elements or structural defects within its pure carbon lattice. These impurities absorb certain wavelengths of light, allowing the remaining, unabsorbed color to be seen by the eye.
Natural yellow diamonds are the most common of the Fancy Colors, with their hue caused by nitrogen atoms incorporated into the crystal structure. These nitrogen atoms form clusters that absorb light in the blue-violet part of the spectrum. The resulting color is the complementary yellow light that is transmitted through the stone.
Blue diamonds are created by an entirely different element: boron. Boron atoms substituting for carbon atoms within the lattice absorb red and yellow light, which allows the blue wavelengths to pass through. This unique chemical composition also makes natural blue diamonds electrically conductive, a property not found in most other diamonds.
Pink, red, and brown diamonds do not owe their color to a chemical impurity but rather to a physical distortion of the crystal structure. Extreme pressure deep within the Earth causes the carbon atoms to shift, creating lattice defects known as plastic deformation. These structural irregularities selectively absorb green light, resulting in the pink, red, or brown appearance of the stone.
Green diamonds acquire their color through exposure to natural radiation from nearby uranium or thorium deposits in the Earth’s crust. This natural bombardment creates a specific type of structural defect, known as a color center, which absorbs red and yellow light. Since the radiation penetration is often shallow, the color in natural green diamonds may be concentrated only near the surface, sometimes appearing as a surface stain.
Industrial Processes for Color Enhancement
Because naturally colored diamonds are scarce, industrial techniques have been developed to enhance or alter the color of less desirable stones. These methods mimic the extreme conditions of the Earth’s mantle or the effects of natural radiation. Such treatments transform brownish or off-color diamonds into vibrant, market-ready gems, making fancy colors more accessible.
High-Pressure High-Temperature (HPHT) Treatment
One of the primary modern methods is High-Pressure High-Temperature, or HPHT, treatment. This process places a diamond in a chamber that subjects it to intense heat, often exceeding 3,600 degrees Fahrenheit, and immense pressure, up to 60,000 atmospheres. The goal is to repair the crystal lattice defects that cause an undesirable brown tint in the original stone.
The HPHT process permanently alters the color centers within the diamond, often transforming brownish stones into colorless or near-colorless grades. When applied under specific conditions to diamonds containing nitrogen, it can produce stable yellow, greenish-yellow, or sometimes blue and green hues. This treatment is considered permanent because it fundamentally changes the diamond’s internal structure.
Irradiation and Annealing
Another widely used technique is a two-step process involving irradiation followed by annealing. The first step, irradiation, uses high-energy particles like electron beams or neutron bombardment to physically displace carbon atoms, creating new color centers. This initial step typically results in a blue or green color throughout the diamond.
The diamond is then subjected to a controlled heating and cooling process called annealing. This heat treatment stabilizes the newly created color centers, causing the color to shift from the initial blue or green to shades of yellow, orange, or pink. While stable under normal conditions, the color from irradiation and annealing can be sensitive to extreme heat, such as that encountered during jewelry repair.
Surface Coating
A third, less common method is the application of a thin-film surface coating. This involves depositing a microscopic layer of colored material onto the diamond’s surface to give it a temporary hue. Coatings are the least stable of the enhancement methods and are generally easy to detect because the color is confined to the exterior and may show signs of wear or damage.
Distinguishing Natural from Treated Color
The difference in value between a naturally colored diamond and a color-treated one necessitates reliable methods for distinguishing their origin. Gemological laboratories employ advanced scientific instruments to determine whether the color is inherent or the result of human intervention. Visual inspection and sophisticated light analysis are the main tools used.
Gemologists primarily rely on spectroscopy, which analyzes how the diamond absorbs light across the visible and non-visible spectrum. Color-causing elements or defects leave distinct “fingerprints” in the absorption spectrum. For instance, the presence of a sharp absorption line at 595 nanometers is a tell-tale sign that a diamond has been irradiated and subsequently annealed.
Another important indicator is the distribution of color, known as color zoning. Naturally colored diamonds typically exhibit angular or irregular color patterns that reflect the stone’s original growth structure. This uneven distribution is visible under magnification.
In contrast, color caused by laboratory treatment often shows characteristic patterns. Irradiated diamonds may display a uniform color distribution or, if treated after cutting, a concentration of color around the culet or girdle, sometimes called an “umbrella effect.” Visual inspection under magnification can also reveal physical signs of treatment, such as minute surface stains from radiation or chips in a coating layer.