When the atoms of carbon in diamond are arranged in a perfect formation, when the crystal lattice is perfect, the stone is colorless. This perfect diamond has a band gap of 5.6eV which can be bridged only by ultraviolet light. Such a perfect diamond therefore absorbs light only beyond the ultraviolet and these stones are colorless, and graded as pure white. More usually, impurity atoms of nitrogen, boron, manganese, or iron may be present in the concentration of a few parts per billion. These impurities are different in size from carbon and so distort the crystal lattice and cause defects levels within the band gap, leading to absorption in the visible regions. In contrast to colored stones like sapphire and ruby, the color is not related to the impurity, but to the defect in the crystal lattice arrangement. The diamond then appears in pale shades of yellow, brown, green or even blue. About seventy years ago, these pale blue diamonds were found in the Jaegersfontein mines in South Africa and are called Blue Jaegers. Slightly yellow and brown stones are the most common and therefore may be up to ten times cheaper than the best white stones.
In contrast to sapphire and ruby, impurity elements that cause color in diamond cannot be removed. Those who claim to bleach diamonds by heating with secret powders are deceiving the public.
Nuclear particles can knock carbon atoms of the diamond crystal lattice out of their place and thereby change the properties of the stone permanently. Irradiation of diamond by atomic particles results in a change in the color of the stone. The user of atomic radiation to improve diamonds has proved to be profitable. Poor color diamonds are not stable and remain with the dealers as dead stock. If these were treated to become novel colors like green, blue, pink or red and retailed with proper marketing and sales publicity, it could open up an entirely new market for jewelry. Soon after the discovery of radioactivity by Becquerel and Curies, scientists investigated the effects of newly discovered atomic rays on precious stones. Sir William Crookes found in 1904 that when exposed alpha rays from radium salts, diamonds turn green. These stones are housed in British Museum in London and their green color is evidence that the color is permanent. The diamonds are, however, radio active and even prolonged chemical washing did not remove the activity.
When the cyclotron was invented, diamonds were exposed to high energy beam from this machine. Dark green ‘cyclotroned’ diamonds resulted from the irradiation. But even at the high energies at which they emerge from the machine, the particles are stopped at a very shallow depth. The deep color is confined to near the surface and may be detected by the so called umbrella effect. Since the a great deal of research has been done on the irradiation of diamonds and other gemstones with beams of various particles and rays such as electromagnetic radiation (ultraviolet light, x-rays and gamma rays), electrons and heavier particles like neutrons, protons, and alpha particles.
Electromagnetic radiation has no effect on diamonds. X-rays and ultraviolet light shining on diamonds make some of them fluoresce, but there is no permanent change. Gamma rays do not color diamonds because they are not able to displace atoms in the diamond lattice, though it has been reported that they cause some increase in the luster. This could happen because there is a temporally shift in the electrons from one position in the crystal to another. Gentle warming or exposure to sunlight reverses any effect that is produced.