The diamond type classification is a scientific method for categorizing diamonds based on the level and type of chemical impurities they contain. In the 1930s, scientists first began using two types to describe the chemical composition and atomic structure of diamonds based on the presence or absence of nitrogen impurities: Type I and Type II. These two types are now further subdivided into Type Ia, Type Ib, Type IIa, and Type IIb based on the arrangement of nitrogen atoms (isolated or aggregated) and the presence of boron impurities.
These impurities should not be viewed as a negative feature; rather, they make each diamond exceptionally unique. These impurities are not similar to defects in a stone and are not even detectable by the human eye, even under a microscope. Since these impurities are measured at the atomic level within the crystal lattice of carbon atoms, unlike inclusions, determining a diamond’s type requires infrared spectroscopy. A diamond can be a mixture of different types. Natural diamonds are often mixtures of Type Ia and Ib, which can be identified using Fourier-transform infrared spectroscopy (FTIR).
Why is Diamond Typing Important?
Diamond typing is a simple method for categorizing diamonds based on their color and physical properties. This is important because it helps form the foundation for identifying natural, synthetic, and treated diamonds (color change by heat/irradiation). Diamonds are essentially made of pure carbon. However, many diamonds also contain trace elements such as nitrogen or boron, which are naturally acquired during their formation.
In other diamonds, trace elements are added as a result of treatment or laboratory synthesis. They also contain other defects in the carbon atom lattice, which scientists refer to as lattice defects or optical defects. These trace elements and other defects are important because they cause various colors as well as fluorescence reactions under ultraviolet light in diamonds. The presence or absence of these defects, their amount, and their arrangement within the crystal lattice can affect the appearance of the diamond.
Type I
Type I diamonds are the most common type, containing nitrogen atoms as their primary impurity, typically at a concentration of 0.1%. Type I diamonds absorb light in both the infrared and ultraviolet regions from 320 nm. They also have characteristic fluorescence and visible absorption spectra.
Type Ia
Type Ia diamonds make up about 95% of natural diamonds. Nitrogen impurities, up to 0.3% (3000 ppm), are clustered within the carbon lattice and are relatively widespread. The nitrogen absorption spectrum can cause the diamond to absorb blue light and appear pale yellow or nearly colorless. In addition to yellow, Type Ia diamonds from certain mines can exhibit other colors such as pink, purple, orange, red, brown, green, or blue.
Most Type Ia diamonds are mixtures of IaA and IaB. These diamonds belong to the Cape series, named after the diamond-rich region formerly known as the Cape Province in South Africa, where the deposits are predominantly Type Ia.
- Type IaA, where nitrogen atoms are paired and do not affect the diamond’s color as Type IaA stones do not absorb visible light.
- Type IaB, where nitrogen atoms are grouped in larger aggregates, surrounding a vacancy (a missing carbon atom). Type IaB may give the diamond a brownish or yellowish tint.
There is also a defect involving three nitrogen atoms called the N3 center, which is not specifically related to diamond type but is one of the most common natural color centers in Cape series diamonds. These centers consist of a vacancy surrounded by three nitrogen atoms and are formed along with other nitrogen aggregates that actually absorb visible light. As a result, the color of these stones may have a yellowish or even brownish tint. They often exhibit blue fluorescence.
Type Ib
Type Ib diamonds make up about 0.1% of all natural diamonds and are much rarer than Type Ia stones. They contain up to 0.05% (500 ppm) nitrogen, but the distribution is more dispersed. Nitrogen atoms in the Type Ib diamond structure are isolated rather than clustered. Type Ib diamonds absorb green and blue spectra and have a more intense or darker yellow or brown color than Type Ia diamonds. The stones have an intense yellow hue or sometimes brown. Most of the fancy-colored diamonds on the market and rare canary diamonds belong to this type, making up only 0.1% of known natural diamonds. The visible absorption spectrum is gradual, with no sharp absorption bands. Almost all synthetic HPHT diamonds are Type Ib.
Type II
Type II diamonds contain no measurable nitrogen impurities and are rarer than Type I. Type II diamonds, unlike Type I, absorb in a different region of the infrared spectrum and transmit ultraviolet light below 225 nm. They also have different fluorescence characteristics. The crystals found are usually large and irregularly shaped. Type II diamonds formed under extremely high pressures over longer periods.
Type IIa
Type IIa diamonds constitute about 1-2% of natural diamonds (1.8% of gem-quality diamonds). These diamonds are almost or entirely free of impurities and are therefore usually colorless and have the highest thermal conductivity. They are very transparent in ultraviolet light up to 230 nm. Occasionally, as Type IIa diamonds are pushed toward the Earth’s surface, pressure and tension can cause structural abnormalities through “plastic deformation” during the growth of the tetrahedral crystal structure, leading to defects. These defects can impart yellow, brown, orange, pink, red, or purple colors to the gem.
Type IIa diamonds can have their structural deformations “repaired” through a high-pressure, high-temperature (HPHT) process, removing much or all of the diamond’s color. Type IIa diamonds constitute a large percentage of Australia’s production. They are the most sought after by collectors. Often referred to as Golconda diamonds, after one of the first diamond mines discovered in India, which supplied the majority of this type. Many famous large diamonds, such as the Cullinan, Koh-i-Noor, Lesedi La Rona, and the Pink Star, are Type IIa. Synthetic diamonds grown using the CVD process also typically belong to this type.
Type IIb
Type IIb diamonds make up about 0.1% of natural diamonds, making them one of the rarest and most valuable natural diamonds. In addition to having very low levels of nitrogen impurities comparable to Type IIa diamonds, Type IIb diamonds contain significant amounts of boron impurities. Boron absorption spectra cause these stones to absorb red, orange, and yellow light, giving Type IIb diamonds a light blue or gray color, although specimens with low levels of boron impurities can also be colorless. Type IIb diamonds show a distinctive infrared absorption spectrum, with gradually increasing absorption toward the red end of the visible spectrum.
Green diamonds, whose color is obtained from exposure to varying amounts of ionizing radiation, are not limited to a specific type. Most of the gray-blue diamonds from Australia’s Argyle mine are not Type IIb but Type Ia. These diamonds contain high concentrations of defects and impurities (particularly hydrogen and nitrogen), and the origin of their color is still not fully understood.
