The main impurity found in diamonds is nitrogen. The atoms become trapped in the diamond’s crystal lattice during its natural formation, causing its natural coloring to be altered. The nitrogen can be present inside the lattice in different forms: as single atoms, as different kinds of small aggregates (2 or 3 atoms), or as larger clusters. The different forms cause the diamonds to change their spectroscopic properties, namely to have different colors by changing the light absorption and refraction/dispersion properties of the stones. It is widely agreed that the incorporation of nitrogen in the diamond structure leads to a yellow to brown color in its appearance.
The presence of nitrogen in a diamond is a determining factor in regards to the category it belongs to. Type I diamonds contain nitrogen whereas Type II diamonds do not. It is acknowledged today that Type II diamonds is not a definitive category as it is too inclusive. There is a further division of diamond types, as Type I diamonds fall into two subcategories: Type Ia diamonds and Type Ib diamonds. Each subcategory correlates to the amount of nitrogen found in the diamond. Type II diamonds are also divided into subcategories: Type IIa diamonds and Type IIb diamonds. Diamonds without nitrogen, Type II, are extremely rare and only account for 1% of all diamonds found.
The diamonds that are defined as Type Ib have the nitrogen atom structure in the crystal as seen in figure 11a. It is namely a single nitrogen atom, N, surrounded by carbon C atoms. A typical Type Ib diamond is seen in figure 11b.
|
|
|
|
Figure 11a: Nitrogen Contamination in Type Ib Diamonds |
Figure 11b: A Photograph of a Typical Type Ib Diamond |
These diamonds have a strong yellow color as well as a strong absorption in the Infrared region. The diamonds that are defined as Type Ia have the 2 adjacent nitrogen atoms in the crystal as seen in figure 12a. It consists of a pair of nitrogen atoms, N-N, surrounded by carbon C atoms.
These diamonds are transparent in the visible spectrum, having no color. (See figure 12b) However, they also have a very strong absorption in the Infrared region.
|
 |
|
|
Figure 12a: Nitrogen Contamination in Type A Diamond Aggregates |
Figure 12b: A Photograph of a Typical Type A Diamond |
The nitrogen contaminated diamonds that are defined as type N3 have 3 nitrogen atoms in the structure of the diamond crystal as seen in figure 12a, and are surrounded by the carbon atoms of the diamond. In figure 13a one of the positions of the carbon atoms is missing, creating a vacancy (denoted as V in the figure). As in most nitrogen-contaminated diamonds, this structure also has a strong absorption in the Infrared region but its weak absorption in the visible range results in a light yellow color known as Cape Yellow. (See right image.)
|
|
|
|
figure 13a: Nitrogen contamination in diamond type N3 |
figure 13b: A photograph of a typical Cape Yellow diamond |
The diamonds that are defined as type H2 (figure 13a) or H3 (figure 13b) have either a single nitrogen atom adjacent to a vacancy, V (type H2), or two nitrogen atoms adjacent to a vacancy, V (type H3) structure in the crystal as seen in figure 13. Basically a single nitrogen atom, N, is surrounded by carbon C atoms.
|
 |
|
Figure 13: Nitrogen Contaminations in Type H2 (NV) Diamonds (a) and H3 (NVN) Diamonds (b) |
Atoms might form large aggregates of nitrogen atoms in diamonds containing high concentrations of nitrogen. These atoms consist of platelets of nitrogen, which are colorless in the visible region, but again, have a strong absorption of light in the Infrared region.
