Update on Synthetic Diamonds
by Heidi Ruffner, Ph.D.
While attending the Tucson Gem and Mineral Show last February, I enrolled in two gemstone courses offered by the Gemological Institute of America (GIA): “Identifying Challenging Synthetics” and “Synthetic Diamonds”. I found the information and hands-on identification work in both courses to be extremely helpful and well worth the modest registration fees ($79 for each course). This issue, I will relay some of the important information regarding synthetic diamonds.
Synthetic diamonds are now commercially available, albeit in limited quantities and colors. Nearly all of the synthetic diamonds that have reached the marketplace are either yellow or yellow-brown in color. Several dealers at the February 1997 Tucson Gem and Mineral Show offered such diamonds for sale. In addition, two Russian synthetic red diamonds (color-treated) have recently been submitted by independent dealers to the major gemological laboratories for identification and analysis (i.e. synthetic reds are out there too!).
Colorless synthetic diamonds are not yet commercially available, although DeBeers has manufactured some prototypes. Some of these prototypes have been sent to GIA for examination and will also serve as standards and educational aids. My diamond sources tell me that these “colorless” synthetics are actually light yellow in color (I, J, or K color on the GIA diamond color grading scale). Achieving a true “colorless” synthetic diamond (D, E, or F color) has not yet been achieved. True colorless synthetics will probably not be commercially available for quite some time.
Synthetic diamonds are, of course, crystallographically and chemically identical to their natural counterparts. Therefore, many of the standard tests for physical and optical properties yield the same results for both synthetic and natural diamonds. The growth processes, however, are significantly different, and the different growth features can establish the source. If these growth features are present in a cut diamond, then these features can serve to ascertain the stone's origin. The table below lists some properties that all diamonds have, regardless of origin, and also some growth characteristics that enable the identification of a diamond as either natural or synthetic.
Properties of Natural and Synthetic Diamonds
Hardness = 10
Refractive index = 2.42 (off the scale for most gem refractometers).
Dispersion = 0.044
Internal inclusions such as feathers, clouds, veils, pinpoints, etc.
Thermal conductivity (standard test used to differentiate diamonds from other colorless, high refractive gemstones, such as cubic zirconia).
Natural Diamonds: Positive Identification Features
Included crystals (also known as “carbon spots”).
Indented trigons (natural triangular shaped external features on diamond crystals). Note: raised trigons may be associated with either natural or synthetics.
Occurs in all colors, including colorless. Note: expect this to change as soon as the synthetic diamond technology matures!
Mottled zoning that does not repeat every 90°.
Usually octahedral crystal habit (uncut).
Synthetic Diamonds: Positive Identification Features
Hourglass-shaped and columnar internal color zoning. Repeats every 90° of rotation.
Radiating octagonal surface pattern on table (raised external feature that remains after polishing).
Geometric patterns (e.g. crosses) seen in fluorescence due to zoning of light emitting ions. Fluorescent pattern is usually green or yellowish green, sometimes orange, but never blue.
Phosphorescence (up to 30 seconds) in near colorless stones.
Cube octahedral crystals (uncut).
Because the supply of synthetic diamonds is still limited, and because the demand for them continues to increase (especially for use as teaching aids and novelty gemstones), their prices remain high. The synthetics currently sell for approximately 90% of the cost of a comparable natural diamond. This relative cost is much higher than that for any of the other synthetically grown gemstones, including rubies, sapphires, emeralds, and spinels. An investment in synthetic diamonds is especially risky, because of the artificially inflated natural diamond prices assigned by DeBeers. I plan to wait a few years before investing in my first synthetic diamond. In the meantime, I expect both quality and selection to increase, and the prices to decrease.
Another newcomer to the diamond market is moissanite, a diamond simulant composed of gem quality silicon carbide. The long-awaited commercial debut of this simulant will undoubtedly cause great consternation, as many of its physical properties are remarkably similar to those of diamond. In particular, moissanite will test “positive” (i.e. as a diamond) on standard “diamond testers” that measure the thermal conductivity of a stone. While current diamond testers will remain useful for differentiating between diamonds and some simulants, such as cubic zirconia, the standard diamond testers will be unable to differentiate between diamonds and moissanite.