Synthetic Diamonds

by GIA
Posted: 5/5/2005 11:44 AM

Editor’s note: The mass media has recently been filled with reports on various synthetic diamonds and their rate of detectability. The Gemological Institute of America’s (GIA) experts have written the following article to clarify some of the controversy surrounding these gem-quality synthetics.

A gem-quality diamond is rare in nature. Even with new discoveries in recent years, existing and developing markets are taxing the supply of natural diamonds. Creation of a diamond in the laboratory from other carbon minerals (e.g. graphite) or carbon-bearing gases [e.g. methane (CH4)] has been studied for about half a century with important progress in the past 20 years. A gem-quality single crystal diamond can be created in the laboratory with two methods. One method is growing a diamond under High Pressure-High Temperature (HPHT) and the other is using the Chemical Vapor Deposition (CVD) method. Although historically the vast majority of synthetics have been used for industrial purposes, with advances in techniques more and more synthetic diamonds are finding their way into the jewelry market. In addition, various methods of artificial treatment are being developed to improve quality of as-grown synthetic diamonds.

HPHT Synthetics
Typical diamond growth conditions with this technique are pressures between 5.0 GPa to 6.5 GPa and temperatures between 1,350°C to 1,800°C. Various types of presses — cubic, belt, BARS — are employed to generate the needed pressure. Inside a graphite furnace, a graphite disk as a carbon source and a diamond seed crystal are placed at the top and at the bottom a metal disk typically of nickel-cobalt is placed. The assembly is specifically designed so that the top side of the molten metallic disk is higher in temperature than the bottom side. The growth of a diamond utilizes the fact that the solubility of the stable diamond phase in the molten metal is lower than that of the metastable graphite. Temperature gradient is another factor that facilitates the growth of a diamond because the solubility of graphite decreases with decreasing temperature. As a result, more carbon atoms are dissolved from the carbon source located at the hotter region and transported to the cooler region that then precipitates on the seed crystal — located at the bottom of the vessel — to form a new synthetic diamond crystal.

Several companies in the U.S. — Gemesis Corp., Chatham Created Gems and Lucent Diamond — are producing HPHT synthetic diamonds for the jewelry market. High-quality crystals up to 3.50 carats and faceted stones as large as 1.50 carats are being produced commercially. The largest-known HPHT synthetic diamond is 34.80 carats grown by De Beers scientists for research purposes in 1992. Due to the very different growth conditions and the environment in which natural diamonds form, HPHT-grown synthetic diamonds display typical cuboctahedral morphology. In contrast, natural diamonds predominantly form in octahedron or distorted octahedrons. Impurities and their concentrations during synthetic diamond growth can be well controlled. While most HPHT synthetic diamonds in the market are type Ib, type IIa and type IIb can also be manufactured by either “blocking” nitrogen from getting into the diamond lattice or intentionally adding trace amounts of boron into the crystal. As-grown HPHT synthetic diamonds can show a large variation in color and saturation including colorless, blue, green, yellow, orange-yellow and yellow-orange. Post-growth radiation with or without annealing can create an attractive pink to red color, in addition to blue and green colors. As a result, HPHT synthetic diamonds are available in almost any color, although the vast majority is yellow (see figure 1). Clarity varies significantly from stone to stone. Some are relatively free of inclusions, but others could be highly included. Equivalent clarity grades range from VVS to I.

Different growth sectors have contrasting behavior in capturing the nitrogen or boron impurity. This results in a distinct color zonation. Also, because of this difference these stones sometimes display a “cross” fluorescence pattern under ultraviolet (UV) radiation. In colorless type IIa HPHT synthetic diamonds, which usually show few identification features, this pattern may also be observed using strong shortwave UV radiation similar to that produced by the Diamond Trading Company’s (DTC) DiamondViewTM (see figure 2). Other useful features for identification include the presence of metallic inclusions, pinpoint clouds and intersecting internal graining. In addition to these visual features, testing with analytical instrumentation provides additional spectral and chemical evidence of their laboratory origin.

CVD Synthetics
In contrast to the conventional HPHT synthetic process, CVD synthetic diamonds are produced at much lower pressure, typically in the region of one-tenth of atmospheric pressure from carbon-bearing gases such as CH4. Commonly, the gas molecules are broken apart in a high-temperature plasma generated using microwaves in a reactor. These chemical reactions deposit layers of a synthetic diamond film as a single crystal over a diamond substrate, which is usually held at temperatures in the region of 800°C to 1,000°C. Much progress has been achieved in increasing the diamond growth rate — ~100 mm/hour — and a relatively large crystal could be created in the laboratory in a reasonably short period of time. In 2003, Apollo Diamond Inc. in the U.S. announced its plan to introduce single crystal CVD diamonds to the jewelry market. The DTC also reported on its work in producing high-quality CVD diamonds for research purposes (Martineau et al. Identification of Synthetic Diamond Grown Using Chemical Vapor Deposition, Gems & Gemology, 2004;40(1):2–25).

CVD synthetic diamond crystals are relatively small in size and usually display a tabular morphology. The largest CVD crystal from Apollo the GIA has examined to date weighs 1.36 carats and is 2.7 mm thick (see figure 3). The largest faceted stone weighs 1.11 carats. Faceted CVD diamonds usually show brown coloration with a large variation in tone. Most of these stones are “pretty clean,” but there are some irregular black inclusions present due to precipitation of nondiamond, carbon. Small fractures were occasionally observed. Equivalent clarity grades ranged from VS to SI. Square and rectangular cuts are common for CVD-grown diamonds in order to achieve the highest yield from the tabular crystals. Very high-quality synthetic diamonds can be created using the CVD technique, as demonstrated by a 1.03-carat fancy deep blue square cut that was VS2 in clarity and a 0.82-carat rose cut with a clarity of VVS1 and E color. These were produced by DTC research for scientific purposes only and the grades are given only for illustration.

When they become available in the marketplace, CVD synthetic diamonds are likely to be difficult to identify using the standard gemological equipment. A brown coloration, the shallow depth of a cut stone and the characteristic strain pattern (see figure 4) of this CVD-grown material may provide clues in many cases. Laboratories equipped with the DiamondView will generally see a strong orangey-red luminescence in the device, which is a good visual indication that the diamond is a CVD synthetic. However, conclusive identification requires the use of advanced spectroscopic methods. The normal production of CVD synthetics is type IIa, most with trace amounts of isolated nitrogen. With photoluminescence spectroscopy, they show strong emissions from nitrogen-vacancy (N-V) centers at 575 nm and 637 nm. A doublet emission at 596 nm and 597 nm, which has never been reported in natural diamonds, is a specific feature of CVD diamonds. Finally, the infrared absorption due to hydrogen (mainly at 3,123 cm-1) and the emission features due to silicon impurity (at 737 nm) appear to be unique to these CVD-grown synthetic diamonds. Color and other physical properties of as-grown CVD diamonds can be significantly improved by HPHT annealing. Brown coloration could be dramatically removed and the stones turn near colorless or only slightly colored after treatment (see figure 5). While HPHT treatment does not change most of the intrinsic identification features of CVD synthetic diamonds, it introduces additional features that indicate it has been subjected to treatment. To date, all synthetics grown by the CVD process have been readily identifiable as synthetic, both those subjected to HPHT treatment and those that have been “as grown.”