Atomic Types of Diamond

Two diamonds can look almost identical yet differ substantially at the atomic level.

One may contain nitrogen in aggregates. Another may contain individual nitrogen atoms. A third may contain so little infrared-detectable nitrogen that standard FTIR analysis does not register it. A fourth may contain boron, which alters the crystal’s optical and electrical properties.

These differences are described by the system of diamond types.

Type is neither a quality grade nor independent proof of origin. It describes particular atomic impurities and how they are arranged within the crystal lattice.

Key lesson Diamond type is an important diagnostic clue, but it is not a verdict on whether a diamond is natural, laboratory-grown, or treated.

The basis of the classification

In gemology, diamond type is determined primarily by:

1. the presence or absence of infrared-detectable nitrogen;
2. the configuration of nitrogen atoms;
3. the presence of boron.

The basic division is:

• type I — contains nitrogen detectable through characteristic infrared absorptions;
• type II — has no nitrogen above the detection threshold of the infrared method used in the relevant spectral region.

Type I is divided into Ia and Ib, and type II into IIa and IIb. Type Ia is further described according to A and B aggregates.

[VISUAL 4.1: Classification tree—I → Ia/Ib; II → IIa/IIb; Ia → A/B/mixed aggregates]

“No nitrogen” is not an absolute statement

Saying that a type II diamond “has no nitrogen” is practical shorthand. More precisely, it lacks enough nitrogen in FTIR-detectable configurations to exceed the method’s detection threshold.

Type IIa is therefore not synonymous with a chemically or structurally perfect crystal. Such a diamond may still contain:

• vacancies;
• dislocations;
• plastic deformation;
• hydrogen-related defects;
• mineral or other inclusions;
• growth traces.

Type is an instrumentally defined classification, not a measure of “purity” in an everyday or clarity-grading sense.

Type Ib: isolated nitrogen

In type Ib diamond, nitrogen occurs predominantly as individual substitutional atoms replacing carbon atoms in the lattice. This form is often called isolated nitrogen or the C center.

Isolated nitrogen strongly affects visible-light absorption and can contribute to colors ranging from intense yellow to orangy yellow.

Natural type Ib diamond is very rare. During the very long thermal history of typical natural diamonds in the mantle, nitrogen can migrate and aggregate, so isolated atoms often develop into more complex configurations.

Type Ib has also historically been common in certain HPHT laboratory-grown diamonds. It can therefore be an important screening signal, but it is not independent proof of laboratory growth.

Type Ia: aggregated nitrogen

The great majority of natural diamonds belong to the type Ia group, although the exact percentage depends on the population being examined. In its general gemological description, GIA gives approximately 95%, while some research populations of colorless and near-colorless natural diamonds show even higher proportions of Ia/IaAB material.

In type Ia diamond, nitrogen occurs in aggregates.

A centers

An A aggregate consists of two adjacent nitrogen atoms substituting for carbon in the lattice. A diamond dominated by this configuration is described as type IaA.

B centers

A B aggregate is a more complex configuration in which four nitrogen atoms are associated with a vacancy. A diamond dominated by these aggregates is described as type IaB.

Mixed IaAB

A natural crystal is often not homogeneous. It may contain both A and B aggregates and is then described as mixed type IaAB.

Such mixing can result from differences in initial nitrogen concentration, growth zoning, and a prolonged thermal history.

[VISUAL 4.2: Isolated N → A pair → B aggregate; schematic, without implying a simple age clock]

Nitrogen aggregation is not a simple geological clock

In simplified form, the sequence can be shown as:

isolated N → A aggregates → B aggregates.

However, the rate of aggregation depends on temperature, time, initial nitrogen concentration, and other conditions. The age of a particular diamond therefore cannot be calculated simply from the ratio of A to B centers without an additional geological model.

The arrangement of nitrogen can carry information about thermal history, but that information must be interpreted in context.

Type IIa

Under the classification criteria, type IIa diamond shows no nitrogen or boron at concentrations detectable by the method used; trace amounts below the relevant detection threshold may still be present.

It can be:

• colorless;
• brown;
• pink;
• purple;
• green;
• gray.

Its color can therefore be associated with other defects, including plastic deformation, vacancies, or color centers that are not part of the type classification itself.

Type IIa is important in identification because it is much rarer than type Ia among natural diamonds. In GIA’s colorless-to-near-colorless laboratory-grown population, type II material is very common and the great majority of such stones are type II. This is a population-level screening signal, not a universal rule for every laboratory-grown diamond.

This creates a useful screening principle:

• colorless type Ia is strongly consistent with the large natural-diamond population;
• colorless type II warrants additional testing.

Natural type IIa diamond does exist, however. Type II is therefore not proof of laboratory growth.

Type IIb and boron

Type IIb has no infrared-detectable nitrogen under the type II criteria, but it contains boron incorporated into the crystal lattice.

Boron can affect:

• blue or grayish blue color;
• the infrared spectrum;
• electrical conductivity;
• luminescence and, in some diamonds, phosphorescence.

Because boron has one fewer valence electron than carbon, it creates acceptor states and can make diamond a semiconductor.

Type IIb occurs in both natural and laboratory-grown diamonds. Neither blue color nor electrical conductivity alone therefore determines origin.

Type is not the same as color

The same color can arise through different mechanisms, and the same diamond type can display several colors.

For example:

• yellow color can be associated with isolated nitrogen, aggregated nitrogen, and other centers;
• brown and pink color can be associated with lattice deformation;
• blue color is often associated with boron, but bluish and greenish tones can have other causes;
• type IIa can be completely colorless or strongly colored.

Diamond type therefore cannot be determined reliably simply by looking at color.

Defects outside the formal type system

The type system does not describe every atomic center in diamond.

Spectroscopy reveals many other defects and centers, including:

• N3;
• H3 and H4;
• NV⁰ and NV⁻;
• GR1;
• SiV;
• hydrogen-related centers;
• deformation-related absorption systems.

Some strongly affect color or luminescence but do not change the basic type designation based on nitrogen and boron.

[VISUAL 4.3: Diamond type versus color center—related but distinct layers of classification]

Type and natural or laboratory-grown origin

No basic type is exclusive to either natural or laboratory-grown diamonds.

Types Ia, Ib, IIa, and IIb can all occur naturally. HPHT and CVD growth can also produce different types, depending on growth chemistry, doping, sectors, and post-growth processing.

Origin identification must therefore not be reduced to statements such as:

“IIa means CVD”

or

“Ib means HPHT.”

Type affects the probability and the next testing step, but a final conclusion requires a combination of growth patterns, inclusions, luminescence, and spectroscopy.

Type and treatments

Diamond type also matters because different defects create different possibilities for altering color.

In certain brown type IIa diamonds, HPHT treatment can reorganize deformation-related defects and reduce the brown component. In diamonds containing more nitrogen, reactions involving vacancies and nitrogen centers can produce different results.

Irradiation and subsequent annealing also depend on which atomic components were present before treatment.

The detailed mechanisms appear in Chapters 39 and 57–63. For this chapter, it is enough to remember that diamond type influences possible reactions but does not prove that treatment occurred.

How type is determined: FTIR

The principal method for determining diamond type is Fourier-transform infrared spectroscopy (FTIR).

FTIR records the absorption of infrared energy associated with lattice vibrations and defects. In the relevant parts of the spectrum, different configurations of nitrogen and boron produce characteristic absorption features.

The instrument does not “photograph an atom.” It measures a spectrum from which the presence of defects is inferred.

The result can depend on:

• instrument resolution and sensitivity;
• beam path;
• signal-to-noise ratio;
• zoning within the stone;
• absorption saturation;
• sample condition and geometry.

In a zoned crystal, a single spectrum may represent a combination of several regions.

Screening is not identification

Because colorless natural and laboratory-grown diamonds have statistically very different type distributions, many screening systems use this difference.

A Pass or Refer result must be interpreted according to the rules of the specific device.

• Pass does not mean that the device reconstructed the stone’s entire history.
• Refer does not mean that laboratory-grown origin has been proven.

Detailed evaluation of screening devices is covered in Part IX.

What type does not reveal

Diamond type alone does not determine:

• clarity grade;
• cut quality;
• market price;
• mine or country;
• age;
• whether color is natural;
• the presence of treatment;
• natural or laboratory-grown origin.

It is one layer of evidence, not the stone’s final identity.

Chapter summary

• Diamond types are based primarily on infrared-detectable nitrogen, its configuration, and the presence of boron.
• Type I contains detectable nitrogen; type II does not contain it above the detection threshold of the FTIR method used in the relevant region.
• Type Ib is characterized predominantly by isolated nitrogen.
• Type Ia is characterized by aggregated forms of nitrogen and is the dominant group among natural diamonds.
• A centers contain nitrogen pairs, while B centers are more complex aggregates associated with a vacancy.
• Nitrogen aggregation carries information about thermal history but is not a simple age clock.
• Under the classification criteria, type IIa has no nitrogen or boron above the relevant detection threshold of the method used, but trace amounts below that threshold and many other defects may still be present.
• Type IIb contains boron and can be electrically conductive.
• Color and diamond type are related but are not the same.
• Natural and laboratory-grown diamonds can belong to the same types.
• Colorless type II is an important screening signal but not proof of laboratory growth.
• FTIR is the principal method used to determine type.
• Type must be considered together with growth patterns, inclusions, luminescence, and other spectroscopic data.

[VISUAL 4.4: FTIR and diamond types—simplified spectral signatures of Ia, Ib, IIa, and IIb]

[VISUAL 4.5: Screening logic—type I/type II as an initial filter, not a final verdict]