(via The Canadian Gemmologist, Volume XIV, Number 1, Spring, 1993) Kurt Nassau writes:
Essentially all the apprehension felt in the diamond trade about the threat of synthetic diamonds is unjustified. It is largely the result of confusion about five groups of synthetic diamond that have quite different growth characteristics. Even scientists themselves often make statements which may be true of one of the five groups of synthetic diamond, but seem to apply to all synthetic diamonds.
As one example, it is true that yellow synthetic diamond can be grown in large, good quality crystals at a relatively low cost by the high pressure technique. Yet this statement does not generally apply to colorless or blue synthetic diamond made by the same process.
As another example, it is true that polycrystalline diamond films can be grown by the low pressure techniques to be a millimeter or more in thickness. Yet it is important to realize that these films are usually neither colorless nor transparent by diamond grading standards when approaching a millimeter in thickness. In addition, millimeter thickness is not achievable at present for single crystal synthetic diamond films even when grown on an existing diamond.
And lastly, there are a whole series of synthetic ‘diamond-like films’ which are too frequently labeled ‘synthetic diamond’, a totally incorrect and misleading usage. These films are usually neither clear nor transparent when of significant thickness. Moreover, they are definitely not diamond, any more than graphite or charcoal could properly be designated ‘diamond’ even though all are forms of carbon.
Confusion also arises from three additional points, which may need clarification. When a crystal grower claims that his crystals have ‘excellent’ quality or are ‘flawless’, he does not mean that they are flawless by gemological standard; usually it means that he can see through the layers or perhaps even read print with the crystal set on the paper, which may not be difficult when the layer is only a fraction of a millimeter in thickness.
The second point involves the definition of ‘diamond’ which is strictly defined as the cubic form of carbon. There is also a hard hexagonal form of carbon, properly designated lonsdaleite ‘hexagonal diamond’ which is erroneous because it means ‘hexagonal cubic caron’. So in the diamond thin-film business ‘diamond’ may not always mean diamond, but may also mean lonsdaleite or amorphous carbon.
Finally, a statement such as ‘a growth rate of one inch per week has been achieved’ does not mean that one inch has been grown; it may mean only that one thousandth of one inch was grown in one-thousandth of a week, that is in ten minutes. But growth rates do not always scale up.
The best way to understand the present rather confusing situation is to consider briefly the high-and low-pressure techniques and the five groups of synthetic diamond products.
Synthetic High Pressure Diamond
The high pressure technique for growth of synthetic diamond was discovered by H Tracy Hall at General Electric in 1955, using temperatures about 2000°C (3623°F) and pressure over one million pounds per square inch. At first, only grit could be made; GE announced gem size crystals in 1970.
Group 1: Synthetic High Pressure Yellow Diamond
Yellow synthetic diamond, colored by traces of nitrogen absorbed from the air during the growth, is readily grown by using a variety of metal solvents; crystals over 12mm across and over 14 carats in weight have been reported. By growing several layers with several small crystals per layer at one time, synthetic diamond slices used as heat sinks for transistors and integrated circuits can be produced economically. Several companies have the capability of doing this, including De Beers, GE and Sumitomo. The gemological properties of such synthetic high pressure yellow diamond have been described for example by Shigley et.al (1987). Identification should present no serious problems.
Group II: Synthetic High Pressure Colorless and Blue Diamond
For the colorless product, nitrogen must be excluded, and boron is additionally used to obtain a blue color. In both instances, special conditions must be used, such as aluminum in the solvent to remove nitrogen. This makes it more difficult to grow many crystals at one time. The characteristics of the products have been discussed by Koivula and Fryer (1984), and Shigley et.al (19860, and identification would probably present no serious problems if and when routine production, and therefore, a standardized product should happen.
Colorless and blue synthetic diamonds suitable for faceting have, indeed, been grown only on an experimental basis so far, and one can estimate that cost of production is not very different from that of natural diamond. To put this statement into perspective, synthetic colored gemstones, such as flux grown emerald, ruby, and so on, appear to be viable at a cost of less than one-tenth that of the equivalent quality natural material, but experience has shown that the market is very restricted except at even lower prices.
Thin Film Carbon Deposition
Various low pressure techniques using carbon-containing gases such as methane or acetylene and temperatures of 1200°C (2192°F) or below are used to grow a variety of types of diamond and other carbon-containing films under non-equilibrium conditions. Even acetylene torches have been used. For reviews of thin film work, see DeVries (1987), Angus and Hayman (1988), and Bachman and Messier (1989).
Group III: Synthetic Polycrystalline Diamond Films
When conditions are carefully controlled, polycrystalline diamond film s can be grown quite rapidly to become a millimeter or more in thickness. When very thin, these would not be readily detected, even by a thermal probe, and would probably not serve any useful purpose in gemology. Weight gain would be negligible and the surface hardness would not be changed significantly.
This last statement may be surprising, but a scratch test applied to a very thin diamond film covering a soft material would merely result in distortion of the substrate, and tearing and detaching of the film. If the film is thick, so that it could give a diamond reaction on a thermal probe ad provide a hard surface for a soft material, it would be visible during a gemological investigation. Moreover, adhesion of such polycrystalline diamond film is extremely poor to substances other than silicon, silcon carbide (moissanite), or diamond itself.
Group IV: Synthetic Single Crystal Diamond Films
This type of film appears to be great concern to some, because it seems to provide the threat of increasing the size of a natural diamond. Single crystal diamond films can indeed be grown on diamond surface by suitable adjustments of the low pressure techniques. Yet, a curious phenomenon occurs: the film only seems to reach a certain thickness, depending on the techniques used. If one tries to force it to continue to grow, then it either stops, turn polycrystalline (when the remarks under Group III apply), or something else happens.
Such films often contain stacking faults and may then be not diamond but lonsdaleite. It is of course possible that someone may discover someday how to keep the films growing, but work on this topic has been underway for over 30 years; a solution just does not seem easy to find. Adhesion is a problem on materials other than those listed above.
One interesting possibility is that a very thin synthetic crystal blue diamond film coating could enhance the appearance of yellow cape-series diamonds. This has been done experimentally (see Fritisch, 1991). Such a film should be as easily detected as a thin blue lacquer layer, by standard gemological testing: the concentration of color in the surface layer can be seen by immersion even in water. The acceptability of such an overgrowth would depend on the nomenclature; it might well be decided that this must be called a ‘natural-synthetic composite diamond.’
Group V: Diamond-like Films
A wide variety of different films which are not diamond but are often so labelled can be deposited quite rapidly and to millimeter thickness by various modifications of the low pressure technique. Some of these are non-crystalline amorphous or glassy films. Some contain significant amounts of hydrogen, up to one hydrogen per carbon, and should then be called hydrocarbon films. Some are partly crystalline but, instead of containing only sp-3 bonding as does diamond, also contain some sp-2 bondings as does graphite. Some have stacking sequences leading to hexagonal lonsdaleite. The materials considered in this group do not have the cube symmetry of diamond.
Additional names that have been used include amorphous carbon, amorphous hydrocarbon, dense (hydro) carbon, hard or superhard carbon, and non-diamond carbon films. All of these names may be appropriate for some such films, but the name ‘diamond’ is definitely incorrect. The hardness may vary from less than 8 to over 9, the color may be black, grey, or near colorless (particularly if very thin), and adhesion is a problem on most materials. It is difficult to visualize any application of such films in the gem field.
The Lessons From Other Synthetics
It is worth examining briefly what happens when usable synthetic gemstones first arrived in the trade. The result was the same each time, whether the product was synthetic ruby in 1905, synthetic sapphire in 1910, synthetic star corundum in 1947, synthetic emerald in 1950, synthetic alexandrite in 1972, and so on. At first there was considerable apprehension, both with respect to the effect on the trade, and also with respect to the ability to distinguish the new synthetic from natural. Each time, this stage was followed by the realization that synthetic could be distinguished from the natural with little trouble, and that synthetic filled a different niche from the natural.
In each instance there was essentially no long term effect on the market for the natural material. It is perhaps worth repeating that synthetics have only been successful in their own niches at a price less than one tenth that of the natural material. Overgrowth of a synthetic over the surface of a natural gemstone has been possible since about 1960, when Lechleitner grew a thin layer of very dark green synthetic emerald over pale natural beryl performs. This has not been a successful product in the market place to date.
In conclusion, there is no significant impact to be expected from synthetic diamond, whether produced in bulk form by the high pressure technique or in the thin film form by the low pressure technique. As always, a steady improvement of growth technology must certainly be anticipated. Significant future breakthroughs are always a possibility and would then change the present status. But they should be of concern only when they happen.
For example, there is little likelihood of D flawless synthetic diamond in carat and larger sizes and at a production cost to make it viable as a synthetic as discussed above for many years to come. After all, someone might just discover next week a new diamond mine containing huge quantities of large flawless stones; while such a discovery is not impossible, it is merely highly improbable.
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