Tuesday, March 10, 2009

Improving Diamonds - Part IV

Diamonds must be heated only in vacuum or else will burn or turn into graphite. Even at low temperatures of 300C to 400C, cracks and other defects that were hidden within the stone appear. Above 1500C, diamond turns to graphite even in vacuum unless it is kept under very high pressures.

Natural pink diamonds are rare, expensive and in demand. Recently it was reported that selected light brown diamonds do sometimes turn pink after irradiation and heating and this led to a scramble to make pink diamonds. Soon afterwards, large quantities of pink diamonds were available in the market; these looked better than their naturally colored counterparts.

A note of caution must be sounded in regard to the treatment of diamonds. The ultimate color that a diamond takes on after irradiation and heating depends entirely on the impurity atoms and their arrangements within the stone. Apparently, if a diamond lacks nitrogen within it, it can be given a pink color. Rough diamonds from various sources are mixed before they come to be cut in India and Sri Lanka. As a result, it is near impossible to predict that color a diamond will finally achieve. Experienced traders claims to be able to select the proper stones for treatment but they are not always successful

One thing must be made very clear. Under no circumstances can an off-color diamond ever be made whiter or colorless! The original color of a diamond, whether it is yellow or brown, cannot be removed by any simple means. One can merely add another color to mask the original shades. The new high pressure, high temperature (HPHT) techniques described later, however, claims to bleach brown diamonds to some extent, but this process it extremely expensive and probably useful only for large stones. Though undesirable colors can be masked by treatment, the clarity of the stone cannot be improved. Those faults are repaired by laser drilling and fracture filling, as described earlier.

Natural diamonds are much more expensive than artificially colored ones. A customer who buys a fancy colored natural diamond has to be assured that he is not being cheated. Some kind of certification by a gem testing laboratory is therefore necessary. In the USA and Belgium, any treatment applied to a diamond must be disclosed to the buyer. When a colored diamond of unknown origin comes into the market, tests have to be made to determine the origin of the color.

Once any induce radioactivity disappears below background levels, it becomes difficult to say if it has been treated. Perhaps electron spin resonance measurements (ESR) and optical absorption spectroscopy done on the stone when it is kept at -160C may determine the history of any treatment. These measurements are expensive and so it is not worth testing a diamond weighting less than 2 carats. The trade assumes that all small colored diamonds have been irradiated, unless proven otherwise.

These sophisticated measurements would then fix the value of the stone. Spectroscopic tests find that the treatment invariably produces as absorption line at a wavelength of 595nm, but heating the stone for several hours at 900C seems to destroy this line. Irradiated green diamonds almost always display a distinct absorption line at 741nm. Yellow treated stones absorbs at 640nm or at 595nm, the two lines accompanied by a 504nm line and sometimes one at 985nm. Only the relative intensity of these lines decides whether the color is natural or not. Therefore a stone with strong absorption lines at 595nm and at 496nm and probably another at 504nm is almost certainly treated, though some naturally colored diamonds show only the 504nm absorption line. However these criteria are not definite as some diamonds show contrary characteristics.


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