By Daniel Howell Ph.D. Chief Scientist and Co-Founder, Diamond Durability Laboratory | 18.12.17
Of all the fancy colours that diamonds can exhibit, green is the only one that can be naturally created once the diamond has been transported to the surface. All other colours are produced while the diamond is deep in the Earth under high temperatures and pressures, either by impurities in the carbon lattice (e.g. yellow, blue) or deformation of it (e.g. brown, red / pink). Natural green colour is the result of radiation damaging the diamond. This can occur when the diamond is still in the volcanic pipe (kimberlite) that brought it to the surface, or even if it has been eroded out from there and found in a river / alluvial deposit. If the diamond resides very near, or in contact with, another mineral or fluid that contains a high concentration of radioactive elements (e.g. uranium) then it can have an affect on it. The radiation emitted by the radioactive minerals or fluids, penetrates the diamond and knocks carbon atoms out of the diamond’s crystal lattice, creating holes or vacancies. It is this defect, or imperfection in the diamond lattice, that causes the absorption of predominantly red light, resulting in us perceiving green colour. Radiation damage can also occur when the diamond is deep in the Earth. However, the high temperatures the diamonds reside at are likely to cause the defects to change, reducing the green colour and potentially changing it to brown or adding a yellow component .
Rough green diamonds are known for often exhibiting just a thin skin of colour. This is the result of two factors, (i) the energy of the radiation causing the damage, and (ii) the length of time the diamond is exposed to the radiation. Using uranium as an example, when it undergoes radioactive decay, it can emit both alpha and beta radiation. Alpha radiation has less energy than beta, and can therefore not penetrate the diamond as far. This means that a diamond subjected only to alpha radiation will have a damaged / coloured surface, while one subjected to only beta radiation will have a deeper penetration of colour than just a thin skin. While the type of radiation affects the physical depth of colour in the diamond, the amount of time the diamond is subjected to the radiation will affect the strength of the colour. This is where the antiquity of diamonds plays a significant role. Radioactive elements can emit radiation for millions or even billions of years. The longer the diamond is irradiated, the stronger the colour will be.
Understanding the processes that cause green colour is integral to determining whether it is natural or man-made. When a cut diamond exhibits a green colouration that aligns perfectly to the facets, then gem labs know the radiation occurred after the diamond was cut, i.e. the colour is man-made. In fact, the radiation of a diamond by a scientist in 1904 (Sir William Crookes) is the first documented case of a man-made diamond colour treatment. He placed a diamond in radium salts for several years. When he retrieved the diamond it had turned green and had a substantial body colour. This type of treatment is very uncommon today, as some of the radioactive material can penetrate into cracks in the diamond, making the diamond itself radioactive . Modern treatments typically use low-energy electron beams to irradiate the diamond, with only surficial affects and no residual radioactivity present in the diamond. Natural green diamonds are not radioactive as the source of the radiation is no longer present.
As with any natural fancy colour diamond, the skill of crafting a beautiful diamond from a piece of rough that likely doesn’t have an even colour distribution, lies firmly in the hands of the cutter. Polishing all of the surface away from a rough green diamond can remove all of the coloured skin, leaving just the white diamond beneath. The best cutters work with the natural shape and colour distribution within the stone to maximize its beauty and rarity.
 Nasdala et al., 2013. Radio-colouration of diamond: a spectroscopic study. Contributions to Mineralogy and Petrology, 165, 843-861
 Nazz and Johnson, 2013. Green diamond, treated with radioactive salt. Gems & Gemology, Spring 2013, 49(1).