Electron bombardment causes a change in the color of diamond as long as they energy of the electrons is enough to knock out atoms from their positions in the lattice. In addition, the energy must also be so high that the electrons penetrate more or less uniformly through the stone, or else the color is confined to the surface. Electrons from a high-energy accelerator knock out atoms to a position intermediate between other atoms, leaving a hole in the lattice. Such hole interstitial pairs are called point defects and they produce a light blue shade, reasonably similar to those very sought after Blue Jaeger. With increasing electron bombardment, this blue gets deeper as the irradiation increases the number of defects. At much higher electron energies, the carbon atom that is knocked out has enough energy to knock out other atoms and a cascade of displacements results. The consequent heavy damage imparts a green color to the crystal.
The most economical method to color a diamond quickly and permanently is to put it into a nuclear reactor inside which large numbers of high speed neutrons fly around. Neutrons do not carry any electric charge and so are not stopped by the nuclei of atoms in the crystal. Neutron irradiation therefore induces a uniform color in the diamond. A neutron being a heavy particle of weight two thousand times that of an electron is able to knock out carbon atoms even at relatively low energy. Again, a high energy neutron that collides with a carbon atom can give it enough momentum to hit other atoms and cause a major series of displacements. Depending on the length of neutron irradiation, the color can be regulated from a blue green tinge to emerald green to a very dark green and on to almost black. Badly flawed stones of poor color have thus been made a lustrous black by heavy neutron irradiation, hiding all the imperfections. The color and its intensity can be just right to offset the original unwanted shade and for this a certain artistic judgment is required.
There have been fears that anything put in to a nuclear reactor would turn dangerously radioactive. Diamond is pure carbon that does not become radioactive though trace impurities present in parts per million may cause some radioactivity. This activity dies down in a few days, but as a safety precaution, neutron treated diamond should be monitored carefully before release. Since slow neutrons are more effective in causing radioactivity, irradiation is best done in a fast reactor or a swimming pool reactor. Rumors that neutron irradiated black diamonds were passed off as natural stones and were a radiation hazards are groundless. In any case, some radioactivity is already present in the human body from the environment and from food and water; compared to this, the activity from colored diamonds is trivial. It is estimated that one has to wear jewelry with 3000 carats of treated diamonds to match the radioactivity already present in one’s body!
After irradiation, the induced color fades to some extent with the diamond is slowly heated at low temperatures of around 300C, but the produced color never completely disappears. If the stone is suddenly heated to a high temperature of about 900C, the defects produced by irradiation move around or migrate in the crystal and finally gather in clusters. Such defect clusters are associated with a rage of shades, from amber, orange and canary yellow to green, blue, red, or gold.