Wednesday, August 29, 2012
Synthetic Diamonds Make Their Mark
The summer 2012 issue of Gems & Gemology, the Gemological Institute of America's (GIA) peer-reviewed quarterly professional journal, offers an extensive analysis of synthetic diamonds based on the work of its researchers and other contributors.
The first article discusses how to distinguish synthetics from natural stones; a second looks at why newer synthetics appear much closer to natural diamonds than those manufactured a few years before. The third article offers an initial evaluation of a new form of synthetic diamond material that has not yet reached the market. All of the articles are available at no cost through the Gems & Gemology iPad app.
In the nearly sixty years since the first synthetic diamonds were created, 2012 may be remembered as the year when they made a major impact in the market for gem-quality diamonds.
Gem-quality colorless synthetic diamonds, which to the unaided eye look identical to natural stones, appeared in the market in commercial quantities early in the year. Controversy arose in the spring when a parcel containing undisclosed synthetics was submitted to a gemological lab for grading. The lab identified the synthetics and announced their findings. Calls from industry organizations for greater disclosure and punishment for those who fail to disclose man-made stones followed immediately.
CVD Synthetic Diamonds from Gemesis
(Wang W., D'Haenens-Johansson, U., Johnson, P., Soe Moe, K., Emerson, E. ,Newton, M., and Moses, T. pp. 80-97)
In March, 2012, Gemesis Corporation, of Sarasota Florida, began marketing colorless synthetic diamonds created by the chemical vapor deposition (CVD) process. CVD synthetic diamonds are produced by using microwaves or other sources of energy to breakdown a hydrocarbon gas, such as methane, inside a vacuum chamber which causes carbon atoms to accumulate in layers on a flat diamond substrate (usually an HPHT synthetic plate), similar to the way snowflakes accumulate in a snowfall.
Gemesis produced its earlier synthetics by the high pressure/high temperature (HPHT) process. Originally introduced by General Electric in 1954, the HPHT process mimics the intense heat and pressure that produces diamonds deep in the earth. The laboratory-based process requires costly equipment to maintain a stable temperature and pressure, and produces mainly yellow-colored stones because trace nitrogen is captured during diamond growth and becomes a coloring agent.
The CVD method, commercialized about a decade ago, is much less costly than HPHT because it works at moderate temperatures and low pressure which requires less expensive equipment. It is also more flexible, because different gases can be used to create a variety of colors or colorless diamonds. Other gases such as oxygen and nitrogen can be added to enhance the quality of the synthetic and improve growth rates.
To better understand and identify the CVD process, GIA researchers purchased 16 stones from Gemesis - 15 of them cut into round brilliants ranging from 0.24 carat to 0.86 carat Most were near colorless and were graded F to G on the GIA 4Cs D to Z scale for diamond color. Three were graded I-J and the largest, 0.90 carat was a rectangle cut graded L. All of the stones were high clarity, VVS or VS. One was graded internally flawless.
The researchers, led by Dr. Wuyi Wang, GIA's director of research and development, subjected the synthetics to an extensive battery of tests including several sophisticated types of spectral analysis to obtain a telltale "signature" for diamonds created by the CVD process. These techniques, which are often the basis for gem identification in laboratories today, require specialized equipment and trained staff to operate the instruments and interpret the resulting data.
In addition, the researchers conducted standard gemological tests, including examinations for identifying visual features of synthetic diamonds such as inclusions, graining patterns and ultraviolet fluorescence reactions. Diamonds have graining or internal strain from the way they crystalize. The growth patterns of lab-grown CVD diamonds are distinctly different from those produced in nature.
Recent Advances in CVD Synthetic Diamond Quality
(Eaton-Magana, S., and D'Haenens-Johansson, U. pp. 124-127)
Recent testing found that the quality of CVD-grown diamonds had improved significantly in the decade since they were introduced. They can now be grown faster and "without color." In the past, CVD synthetics displayed graining patterns not found in natural diamonds, distinctive inclusions and fluorescence. The samples recently examined by GIA showed that separating such stones from natural diamonds is increasingly difficult and now requires advanced spectroscopic techniques because of their high clarity and color. In addition, it was apparent that some CVD synthetic diamonds were treated under high heat and pressure which could remove unwanted colors and may also help improve transparency. As a result, those CVD characteristic features are very difficult to see without advanced equipment.
Despite the difficulty in spotting these stones by conventional means, GIA researchers did find unique spectroscopic signatures including photoluminescence features and UV fluorescence patterns.
In the past decade, CVD producers have found that changing the gasses in the growth chamber and using a "purer" (Type II) synthetic diamond as a seed crystal plate can improve the color of the finished diamond and speed the growth rates by much as five-fold. Also, since many as-grown CVDs have a brownish color, producers have found that high pressure and temperature treatment can improve the color, and can also mask some of the signature visual features of these synthetics that make identification more difficult.
Nano-Polycrystalline Diamond Sphere: A Gemologist's Perspective (Skalwold, E. pp. 128-131)
A new type of synthetic diamond, a sphere of nano-polycrystalline diamond (NPD), was developed in Japan late last year. Unlike natural diamonds or the synthetics mentioned above which are a single crystal, NPD diamond is actually a cluster of tiny crystals, thousands of times smaller than the width of a human hair, packed so tightly together that they are transparent and tougher than a natural diamond. Indeed, the crystals are so tightly bonded to one another that there is no grain that is prone to fracture or break.
The NPD samples examined were brownish yellow in color and some were in perfect sphere shape which is difficult to fashion. The techniques recently developed in Japan allow NPD diamonds to be fashioned into virtually any shape, including a round brilliant cut. However, because NPD diamonds are tougher than natural stones, they can only be shaped by a certain kind of laser cutting. Traditional cutting methods will not work.
Developed for industrial uses, NPD synthetics have not reached the jewelry industry. However, just as with HPHT and CVD synthetics, GIA researchers and laboratory staff have identified the unique properties of this material that should be readily identifiable with laboratory testing.
Synthetic gem-quality diamonds may have made a mark on the industry this year, but the science is keeping up. As techniques for synthesizing diamonds evolve, researchers and laboratory experts at GIA continue to follow developments, cultivating ever-more innovative identification techniques.