Diamonds come in different colours because of variations in their atomic structure. An ideal, colourless diamond is built fully of carbon atoms. Most diamonds however are not ‘ideal’. Their structure often contains atomic impurities. These impurities may be foreign atoms (most commonly nitrogen, but also boron) or vacancies in their atomic structure– places normally occupied by a carbon atom that now are empty.
Nitrogen atoms are the most common foreign atoms found in diamonds, that is because nitrogen is the 6th most abundant element in the universe and accounts for 75% of the earth’s atmosphere. Because of its impact on diamond colour, international grading institutions proposed the following categorisation of diamonds:
- Type I diamonds – diamonds that contain a detectable quantity of nitrogen
- Type II diamonds – diamonds that contain practically no nitrogen.
Type I diamonds are the most common, they are called the cape types by the trade. More than 95% of all polished diamonds fall into this category. Because some of the nitrogen atom groups absorb blue light, these diamonds often have a light-yellow colour, but brownish-yellow or brown is also possible. Depending on the grouping of the nitrogen atoms, type I diamonds are subdivided into:
- Type Ia – where nitrogen atoms are arranged in groups
- Type Ib – where nitrogen atoms are not arranged in groups, but individually spread and isolated in the crystal structure.
Type II diamonds do not contain nitrogen atoms. Since nitrogen is considered the most prominent impurity, this type II diamonds are considered the most atomically pure. They subdivide in the following categories:
- Type IIa – don’t contain neither nitrogen nor other impurities that can influence the colour of the diamond. They are usually colourless. However, because of stress differences between the growth lines, these diamonds can sometimes be brown, pink or even red. Type IIa diamonds are rare and they amount for 1-2% of all diamonds. It is being said that type IIa diamonds are formed far deeper in the earth core than other diamonds.
- Type IIb – diamonds that have boron atoms in their crystal structure. The boron content is usually low. Boron atoms absorb infrared light but also red, orange and yellow visible light. Therefore, type IIb diamonds are blue, grey or nearly colourless. They conduct electrical current. All other types of diamonds are electrically insulating at room temperature. So, a blue diamond which is not electrically conductive is probably artificially coloured. Only 0.1% of diamonds are type IIb diamonds.
|Type I||Type II|
|Type Ia||Type Ib||Type IIa||Type IIb|
|Colour centres||Groups of nitrogen atoms||Isolated nitrogen atoms||No specific colour centres||Boron atoms|