Why Diamonds Have Color

An idealized diamond lattice made only of properly arranged carbon atoms absorbs visible light very weakly. Real diamonds, however, are not ideal crystals.

They may contain:

• atomic impurities;
• vacancies in the lattice;
• complex defects;
• plastic deformation;
• growth zones;
• mineral and other inclusions.

These disruptions account for a large proportion of the colors we see.

The most important idea in this chapter is:

Diamond color is not a single piece of data. It is the optical result of structure, defects, their spatial distribution, and the path of light through the stone.

Selective Absorption

White light contains a broad range of visible wavelengths. If a diamond absorbs some parts of the spectrum more strongly than others, the light emerging from the stone no longer has the same spectral composition.

The eye and brain perceive this altered spectrum as color.

Color is therefore not a “pigment” lying inside a diamond. It arises from the relationship between:

• absorption;
• transmission;
• scattering;
• luminescence;
• the stone’s geometry;
• the light source;
• human vision.

[VISUAL 32.1: White light enters a diamond; selective absorption changes the spectrum emerging from the stone]

The Band Gap and Optical Centers

Diamond has a wide band gap. In a perfect lattice, visible photons generally do not have the appropriate energy for strong electronic absorption.

Defects can introduce new energy states within that gap. Such a site can act as an optical center or color center.

A color center may be associated with:

• a single impurity atom;
• several bonded atoms;
• an atom and a vacancy;
• a group of vacancies;
• a deformed lattice.

One diamond may contain several centers simultaneously. Its observed color may therefore be the combined result of several absorption systems.

Nitrogen: the Most Common Impurity in Natural Gem Diamond

Nitrogen is the most important atomic impurity in a large proportion of natural diamonds, but its presence alone does not mean that a stone will be yellow.

The configuration of nitrogen is crucial.

Isolated substitutional nitrogen—the C center associated with type Ib—absorbs the blue part of the visible spectrum very effectively and can produce a strong yellow or orange-yellow color.

In other diamonds, nitrogen aggregated into pairs and more complex groups over geologic time. Some aggregates do not themselves produce visible color, while more complex defects such as N3 and H3 can contribute strongly to yellow, greenish, or orange appearances.

It is therefore incorrect to simplify the relationship as:

“more nitrogen = a yellower diamond.”

Configuration, concentration, other defects, and optical path all matter.

Boron and Blue Color

Boron can replace a small number of carbon atoms and introduce electronic states characteristic of type IIb diamonds.

In many natural blue diamonds, boron is the primary cause of blue to grayish-blue color and is also associated with electrical conductivity that is not typical of most diamonds.

But blue is not synonymous with boron. Blue, gray, and violet appearances may have other causes, as discussed in detail in Chapter 36.

Vacancies and Vacancy-Related Centers

When a carbon atom is missing from the lattice, a vacancy is created. Vacancies can combine with other atoms and form optically active centers.

Examples include:

• GR1—a neutral vacancy, important in many irradiated green diamonds;
• H3—two nitrogen atoms adjacent to a vacancy;
• H4—a more complex nitrogen-vacancy structure;
• NV⁰ and NV⁻—nitrogen-vacancy centers, important in some pink and red diamonds and in many treated and laboratory-grown examples.

The name of a center does not, by itself, establish its geologic history.

Plastic Deformation

Diamond is extremely hard, but under appropriate pressure and temperature its lattice can deform plastically without macroscopic fracture.

Such deformation can produce:

• dislocations;
• slip zones;
• colored lamellae;
• groups of structural defects.

Plastic deformation is associated with two exceptionally important groups of colors:

• common brown color;
• the vast majority of natural pink and red diamonds, as well as some purple diamonds.

In brown diamonds, groups of vacancies are considered an important cause of broad absorption. In pink and red diamonds, a broad band around 550 nm is crucial, but its exact atomic structure has not yet been conclusively identified.

[VISUAL 32.2: Plastic deformation—dislocations, colored lamellae, and the different optical effects of brown and pink systems]

Natural Radiation and Green Color

High-energy radiation can displace a carbon atom from its site and create a vacancy. In nature, this can occur while a diamond remains near radioactive minerals or fluids in a geologic environment over an extended period.

If the effect is limited to the surface, rough may have green spots or a thin colored zone. If the radiation penetrated more deeply, the color may affect a larger volume.

The same basic type of structural damage can also be produced by artificial irradiation. The presence of a GR1 center or green color alone therefore does not prove that the color is natural.

Black and White Appearances Are Not Simple “Color Centers”

Some diamonds appear black because a large number of dark inclusions, fractures, or other structures absorb and scatter light so strongly that the stone becomes opaque or nearly opaque.

In polycrystalline aggregates, numerous grain boundaries and porosity play an additional role.

White or milky diamonds may appear bright because of intense scattering, for example from numerous tiny inclusions or structural irregularities.

This must be distinguished from the selective absorption that creates classic chromatic color.

Body Color Is Not Fluorescence

Body color is observed while the stone is illuminated. Fluorescence is the emission of light during excitation by an appropriate source, while phosphorescence may continue after the excitation has ceased.

The same defect may participate in both absorption and emission, but the two phenomena are not the same.

A diamond may be yellow and fluoresce blue. It may be blue and phosphoresce in another color. It may have strong fluorescence without a change in its official body-color grade.

Basic luminescence is discussed in Chapter 30; detailed instrumental analysis belongs to Part IX.

Color Is Spatial

Defects are not always distributed uniformly.

Color may be concentrated in:

• growth zones;
• deformation lamellae;
• surface radiation stains;
• localized sectors;
• areas rich in tiny inclusions.

A spectrum obtained along one optical path therefore may not represent the entire volume of the stone.

In a polished diamond, facets further multiply the optical paths. Reflections can make a small colored zone appear in several places, while another zone may remain almost hidden.

[VISUAL 32.3: Growth zoning, deformation lamellae, and surface radiation stains as three different spatial distributions of color]

The Same Center Does Not Mean the Same History

One of the most important principles of identification is that the same color center can exist in several contexts.

NV centers may be natural, produced by treatment, or developed in laboratory-grown material. GR1 may result from natural or artificial irradiation. Boron may be incorporated during natural or laboratory growth.

The statement:

“a particular center has been detected”

is therefore not equivalent to:

“the natural origin of the color has been proven.”

The second conclusion requires a combination of several independent observations, including the spatial distribution of defects, diamond type, growth patterns, spectroscopy, and other laboratory indicators.

[VISUAL 32.4: The same color center in natural, treated, and laboratory-grown contexts; a body of evidence is required for a conclusion]

From Physics to Grading

Physics explains why a stone absorbs light. Grading describes how that stone appears in a standardized procedure.

These are not the same tasks.

Spectroscopy may reveal a 550 nm band. Grading may describe the stone as Fancy Intense purplish pink. One is a physical mechanism; the other is a standardized description of observed appearance.

Chapter 33 therefore moves from the atomic level to the system used to describe fancy-color appearance formally.

Chapter Summary

• Diamond color results from selective absorption, transmission, scattering, and sometimes luminescence.
• Optical centers introduce energy states that an ideal diamond lattice does not have.
• Nitrogen is a common impurity, but its configuration determines its optical effect.
• Isolated nitrogen, N3, H3, and other nitrogen-related centers can produce different yellow and orange appearances.
• Boron is an important cause of blue color in type IIb diamonds, but it is not the only possible cause of blue or grayish-blue appearance.
• Vacancies and vacancy-related centers are important in green, pink, and other colors.
• Plastic deformation is crucial to a large proportion of brown and natural pink/red diamonds.
• The exact atomic structure of the most common 550 nm pink center has not yet been conclusively resolved.
• Black and white appearances often involve absorption and scattering from inclusions, fractures, or numerous tiny structures.
• Body color, fluorescence, and phosphorescence are not the same properties.
• Color may be strongly zoned, so the spatial context of a measurement matters.
• The same optical center may exist in a natural, treated, or laboratory-grown diamond.
• Spectroscopic detection of a center is not in itself proof of natural color status.