ARTIFICIAL COLOURING OF DIAMOND

To the purist any tampering to affect the physical properties of diamond smacks of faking and is to be deplored. Yet there is one aspect of modern diamond jewelry technology which may have future repercussions. It has been established that the colour of a diamond can be changed if it be subject to bombardment by fast atomic particles. A clear transparent diamond can have its colour changed when irradiated in an atomic pile. This change in colour comes about through what is called 'radiation damage'. A bombarding particle strikes an atom which is situated in its normal strict geometrical position in the crystal and knocks it out of position, leaving a 'vacant site' behind. The displaced carbon atom must find a home and does so 'interstitially' between other atoms which are thus distorted from their normal strict geometrical positions by the intruder.
The effect of the intruder is to influence the physical properties, and one of these results is to create an absorption of some of the colours of the spectrum. As a direct result, the crystal acquires a colour which is quite a pure colour.
An atomic pile reactor is a system which creates large numbers of uncharged atomic particles called neutrons, and these are especially efficient in penetrating right through a crystal and knocking out atoms, which in turn also knock out atoms. This is done in depth right through the crystal.
Another way of creating this situation of knocked-out particles is to accelerate electrons to a high velocity with a machine called a Van de Graaff generator. This machine builds up millions of volts, and the electrons accelerated by such fields are also capable of inducing colour in diamond, but this time the colour is on the surface only.
Diamonds exposed to the neutron irradiation available from the atomic pile at Harwell became green. The intensity of the colour depends on the irradiation dosage. A light colour results from a small dosage and a progressive deepening occurs as dosage increases. Beyond this the diamond turns irretrievably black, in fact local regions of graphite have then formed.
If a diamond coloured. green by neutrons is heated, the colour changes since the interstitial atoms can move to new positions because of their increased vibrations induced by heating. Changes to amber and then to yellow appear, depending on both the temperature of heating and the time the heat is applied. The diamond never returns back to its original white transparency, having clearly suffered some permanent damage which cannot be resolved. A yellowish brown always remains at best. Any colour induced, whether green, brown or yellow, remains permanent if the diamond is cooled, at that colour, down to room temperature. Changes only occur whilst the diamond is hot, so in this sense a colour once induced is permanent at room temperature.
Electrons, contrary to neutrons, produce a pure blue colour. Electrons, even if fast, are absorbed in a skin-thin layer of diamond so that the blue coloration from electrons is only 'skin' deep (1/50 inch). Not only is this a mere thin skin colour, but also only that particular face which was exposed to the beam takes on colour. To colour a complex-shaped brilliant by exposure to electrons would be a most complicated matt« because of its elaborate shape and the inclined angles of its many faces.

Natural blue diamonds are very rare and very remarkable, but there is at present no risk of confusing the natural blue diamond with the irradiation blue diamond. The colours are visually very different and certainly also quite different when measured with spectroscopic instruments. Once more, then, we have a situation where the artificial creation can be distinguished from the natural. The man-made color is immediately recognizable and as such will not debase the value of the natural blue diamond. But it might enhance a natural colorless white! It is thus possible that a demand for radiation colored diamonds will arise in future.
The electron-irradiated blue is not stable to heat and turns to green at moderately high temperatures. Natural blues can be heated to 1250° C without any change in color.
There remains another very important point not to be overlooked. Irradiation of pure carbon seems to be harmless, but all diamonds contain some impurities. During the bombardment some of these impurities become radioactive, and this might be long-lived and possibly dangerous to a wearer. If irradiation were to be used in future for coloring diamond stud earrings , it would become imperative to test each such diamond for radiation danger.
Finally, there may possibly be a bonus from irradiating diamond. There seems to be some evidence that irradiation reduces the intrinsic hardness through damaging the atomic groupings. Thus an irradiated diamond might become easier , to fabricate. When one takes into account, however, the high cost of neutron irradiation, it may well be quite uneconomic - | the saving in polishing time might well be insufficient to make up for the cost of irradiation.