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Increasingly one encounters in the trade literature references to the "types" of diamond, such as type Ia or type IIa. This brief note contains a simplified explanation of this terminology.
A "perfect" diamond would be colorless, and would have all of its carbon atoms correctly arranged in a regular crystal structure. However, most all gem diamonds depart slightly from this condition, since they contain small numbers of defects (for example, missing or extra atoms), or foreign atoms (mainly the elements nitrogen and boron, both substituting for carbon) at the atomic level. The presence of these defects and/or foreign atoms can affect some of the physical properties of a diamond, such as its color and its ability to conduct electricity.
As part of an effort to explain observed or measured differences in physical properties among diamonds, scientists in the 1930s devised a simple scheme to categorize diamonds into two main groups, or types ? type I and type II. Historically, these two categories have been distinguished by differences among diamonds in their ultraviolet transparency, and in their infrared spectra. The use of this terminology has continued to the present time because it provides a convenient way to group diamonds that have similar physical properties.
Type I diamonds are the most abundant category in nature. Features in the visible and infrared spectra of type I diamonds have been attributed to the presence of nitrogen atoms in various configurations. These diamonds can be further subdivided into types Ia and Ib, depending on the form that the nitrogen atoms take in a particular diamond.
Type Ia diamonds contain nitrogen atoms in aggregates (that is, two, three, or four (or perhaps more) atoms occupy neighboring positions in the crystal structure). They represent the vast majority of natural gem diamonds (and are typically colorless to light yellow). They frequently display several sharp absorption bands (the "Cape" lines, with the main band at 415 nanometers) in their visible spectra, and they exhibit blue fluorescence to long?wave ultraviolet radiation. More aggregates of three nitrogen atoms lead to greater intensity of the Cape lines, making a stronger absorption of light at the blue end of the spectrum, and in turn, a more intense yellow coloration for the diamond.
Type Ib diamonds also contain nitrogen, but in this case the nitrogen occurs as isolated, individual atoms dispersed in the crystal structure. In this form, the nitrogen gives rise to a gradually increasing absorption of light toward the blue end of the spectrum, but no sharp absorption bands. This pattern of light absorption produces a deep yellow color.
The other main category is represented by type II diamonds, which are thought to contain little or no nitrogen (since they do not show nitrogen?related absorption features in their infrared spectra). Type II diamonds are much less common in nature than type I diamonds.
Type II diamonds that do not conduct electricity are designated as type IIa. These diamonds are very transparent in the short?wave ultraviolet region of the spectrum (they will transmit ultraviolet radiation with wavelengths down to approximately 230 nanometers). Unless they contain inclusions or atomic?level structural defects that would cause light absorption (and thus possibly produce coloration), type IIa diamonds are usually colorless. On occasion they can also be gray or light brown, light yellow, or light pink (they could also be light blue to light green due to radiation exposure). Some large, historic colorless gem diamonds are type IIa.
Type II diamonds that do conduct electricity are called type IIb. In contrast to the other categories, these diamonds exhibit a distinctive infrared absorption spectrum, and contain boron as a foreign element. The presence of boron produces a gradually increasing absorption of light toward the red end of the spectrum, and thus type IIb diamonds are blue or grayish blue in color.
In many articles where diamond types are mentioned, it is generally implied that one diamond contains only one diamond type. This is not entirely correct. The occurrence and aggregation state of the nitrogen atoms can vary from point to point in a single crystal, which means that most diamonds are actually mixtures of different types. For example, many natural type Ib diamonds contain a small component of type Ia, which can be recognized by features in the infrared spectra. The description of the type of a particular diamond usually refers to the overall result determined from analyzing the diamond as a whole.
A general understanding of diamond types is useful for several reasons. As indicated above, these types are more or less common among natural gem diamonds. The percentages of various types tend to be different for synthetic diamonds, since it is possible to some extent to control the kinds and amounts of foreign elements during the synthesis process (so that diamonds can be grown) for specific industrial applications. In addition, the various diamond types respond differently to laboratory treatments, such as irradiation, heating, and heating at high pressure (HPHT). Establishing diamond type is an important part of the gem identification process, since this information is very useful in helping to recognize treated and synthetic diamonds.
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