How diamond type can affect colour

Diamonds can be classified into type 1 diamonds and type 2 diamonds with further classification of type 1 a, type 1 b and type 2 a and type 2 b.

One of the determinants of the colour of a diamond is dependent on the particular characteristics of the trapped foreign objects. In the case of the aforementioned nitrogen (N), the trapped nitrogen atoms, if in large, even-numbered aggregates greater than pairs, will absorb the wavelengths of light that produce a light yellow or brown colour. These diamonds that hold nitrogen as their primary impurity fall into the category of Type I diamonds, which make up 98% of all diamonds. The colour blue is produced as a result of the presence of the elements boron (B) or hydrogen (H), as these colours absorb the light wavelengths that produce the colour blue. Diamonds with boron as their impurity fall into the category of Type II diamonds.

It may appear that all diamonds are alike, at least in their basic make-up; however, it is important to be aware that there are some notable differences within diamonds at the atomic level. This main distinction is made between diamonds that contain nitrogen and those diamonds that do not contain nitrogen. This is illustrated in the splitting of diamonds into two main diamond categories: Type I and Type II. These two categories are differentiated on the atomic level, and so certain tools, such as a spectrograph (a device that measures the amount of infrared light that is absorbed by the diamond) are required even by an experienced graduate gemologist to place them within a Type category.

Type I Diamonds

In the 1930's it was discovered that about 98%-99% of all found diamonds contained nitrogen (N) as their primary impurity. This nitrogen is detected using a spectroscope, as the nitrogen absorbs infrared light. This impurity usually occurs in one of two ways, either in large groups (aggregates), of nitrogen, or as single nitrogen atoms scattered within the diamond's crystal lattice. These two forms of impurity are further broken down into two sub-groups: Type Ia and Ib.

Type Ia diamonds are those diamonds that contain nitrogen in aggregates. When these aggregates occur in pairs within the diamond lattice (A aggregates) or as aggregates of four (4) nitrogen atoms (B aggregates) they do not absorb any of the wavelengths of visible light. When nitrogen occurs in groupings of three (known as N3 center) however, the nitrogen groups absorb the visible light within the blue end of the spectrum, causing a yellowish colour to appear in the diamond. This means that the nitrogen in Type IaA-aggregate and IaB-aggregate diamonds does not affect a diamond's colour, whereas the nitrogen of Type IaN3 center does affect the diamond's colour, saturating it with the yellowish tint we are familiar with.

Type Ib diamonds are far less common than Ia, occurring in less than .1% of all naturally occurring diamonds. Type Ib diamonds are diamonds that contain single nitrogen atoms scattered throughout the diamond's crystal lattice framework. It is this scattering of single nitrogen atoms, allowing the absorption of visible light in the blue end of the spectrum that gives rise to the intense yellow colour found in the true Canary yellow diamonds (rare and valuable Fancy coloured diamonds). This atomic formation can also give rise to browns and yellowish -green diamonds.

Type II Diamonds

In Type II diamonds, there is no sign of nitrogen present when viewing the diamond's light properties on infrared light with a spectroscope, as there is little absorption of infrared light occurring within these diamonds. This does not mean that absolutely no nitrogen atoms are present within the diamond's crystal lattice. The spectroscope's resultant findings simply indicate that nitrogen does not appear in a large enough quantity to have any affect on the diamond's optical properties. Type II diamonds are also split into two categories, Type IIa and Type IIb. These sub-groups are separated with Type IIa diamonds not capable of conducting electricity and Type IIb diamonds that are able to conduct electricity.

Type IIa diamonds are colourless, unless they have an inclusion or defect in the crystalline structure that would allow light absorption to occur. Diamonds of this type can be gray- brown, yellow, pink, light blue, or light green (with these two last arising from radiation exposure), but more often these are the perfectly colourless diamonds that most people are looking for. This is due to the fact that their structure does not easily absorb short-wave ultraviolet light wavelengths, but instead allows the light to pass through. Such famous diamonds as the Cullinan (the world's largest cut diamond, found in the South African Premier Mine in 1905) and the Koh-i-Noor (Urdu for "Mountain of Light"), which was found in India and now rests in the Tower of London, are type IIa diamonds.

Type IIb are diamonds that are electrically conductive, and the most distinguishing feature is the presence of boron (B) in this type. Boron is not present in Type IIa diamonds, and serves as the primary difference between the sub-groups. The presence of boron within this diamond type causes almost all of them to be either blue or bluish gray. Boron, as a foreign element in Type IIb diamonds, provides the blue colour due to the boron's absorption of light towards the red end of light's colour spectrum.