Synthetic Diamonds and Simulants

Synthetics

The jewelry industry uses special terms for manufactured and look-alike gemstones: synthetic and simulant. The differences between them are subtle, but very important.

Synthetic refers to a manmade material with the same chemical composition, crystal structure, and optical and physical properties as the natural gem material. Synthetic diamonds contain carbon atoms and they are arranged the same way as in naturally occuring diamonds.

Synthetic industrial diamonds, which are used as abrasives and in cutting tools, have been manufactured since the mid-1950s. Gem-quality synthetic diamonds have been commercially produced in limited quantities since the mind-1990s.

Simulants

Materials that simply look like natural gems are called simulants or imitations, "substitute" is an older term for the same thing. The materials can be either natural or manmade.

A number of materials have been used as diamond simulants. Some of the more common ones are glass, zircon, and colorless synthetic spinel. YAG (Yttrium Aluminum Garnet) and GGG (Gadolinium Gallium Garnet) are sometimes used and were both developed in the late 1960s.

Today, those simulants have been almost entirely replaced by CZ-synthetic cubic zirconia. It has "synthetic" in its name because cubic zirconia actually exists in nature, but in crystals too small for use in jewelry.

Synthetic moissanite, introduced in the late 1990s, is another modern diamond simulant.

Manmade Diamonds are rapidly becoming more popular as their manufacturing processes are being refined.

Summary: 

Making Synthetic Diamonds

There are currently two methods of synthesizing diamonds.

High pressure - high temperature (HPHT) methods subject graphite to conditions similar to those under which natural diamonds are formed in the Earth’s mantle. Most large, gem quality synthetic diamonds are grown using variants of this method. The most successful involves growing the diamonds in a ‘flux’ of molten metal alloy.

There are also vapour deposition methods which operate at lower temperatures and pressures, and are usually used to produce thin films for industrial applications.

Researchers at the Carnegie Institution's Geophysical Laboratory have learned to produce 10-carat, half-inch thick single-crystal diamonds at rapid growth rates (100 micrometers per hour) using a chemical vapor deposition (CVD) process. This size is approximately five times that of commercially available diamonds produced by the standard high-pressure/high-temperature (HPHT) method and other CVD techniques. In addition, the team has made colorless single-crystal diamonds, transparent from the ultraviolet to infrared wavelengths with their CVD process.

"High-quality crystals over 3 carats are very difficult to produce using the conventional approach," commented Dr. Russell Hemley who leads the diamond effort at Carnegie. "Several groups have begun to grow diamond single crystals by CVD, but large, colorless, and flawless ones remain a challenge. Our fabrication of 10-carat, half-inch, CVD diamonds is a major breakthrough." The results were reported at the 10th International Conference on New Diamond Science and Technology, Tsukuba, Japan, on May 12, and will be reported at the Applied Diamond Congress in Argonne, Illinois, May 18.

Most HPHT synthetic diamond is yellow and most CVD diamond is brown, limiting their optical applications. Colorless diamonds are costly to produce and so far those reported are small. This situation limits general applications of these diamonds as gems, in optics, and in scientific research. Last year, the Carnegie researchers found that HPHT annealing enhances not only the optical properties of some CVD diamond, but also the hardness [1]. Using new techniques, the Carnegie scientists have now produced transparent diamond using a CVD method without HPHT annealing.

To further increase the size of the crystals, the Carnegie researchers grew gem-quality diamonds sequentially on the 6 faces of a substrate diamond plate with the CVD process. By this method, three-dimensional growth of colorless single-crystal diamond in the inch-range (~300 carat) is achievable.

The standard growth rate is 100 micrometers per hour for the Carnegie process, but growth rates in excess of 300 micrometers per hour have been reached, and 1 millimeter per hour may be possible. With the colorless diamond produced at ever higher growth rate and low cost, large blocks of diamond should be available for a variety of applications. "The diamond age is upon us," concluded Hemley.