HPHT-Grown
Synthetic Diamonds
         Almost a decade ago, Shigley et al. (1995) published a comprehensive chart to illustrate the distinctive characteristics of yellow, colorless, and blue natural and synthetic diamonds. The accompanying article reviewed synthetic diamond production at the time, and discussed how the information presented on the chart was acquired and organized. It also included a box that provided a “practical guide for separating natural from synthetic diamonds.” The chart was distributed to all Gems Gemology subscribers, and a laminated version was subsequently made available for purchase.
           Since that time, and especially within the past several years, the situation of synthetic diamonds in the jewelry marketplace has become more complicated. Lab-created colored diamonds are now being produced in several countries (including Russia, the Ukraine, Japan, the U.S., and perhaps China and elsewhere), although the quantities continue to be very limited. And today they are being sold specifically for synthetic diamonds seen occasionally in trade publications and other industry media. Recent inquiries to three distributors in U.S. Chatham Created Gems of San Francisco, California; Genesis Corp. of Sarasota, Florida; and Lucent Diamonds Inc. of Lakewood, Colorado indicate that their combined production of crystals is on the order of 1,000 carats per month (mainly yellow colors), a quantity that does not meet their customer demand.
           The synthetic diamonds currently in the gem market are grown at high pressure and high temperature (HPHT) conditions by the temperature-gradient technique using several kinds of high-pressure equipment (belt, tetrahedral, cubic, and octahedral presses as well as BARS apparatuses), and one or more transition metals (such as Ni, Co, and Fe) as a flux solvent/catalyst. Typical growth temperatures are 1350-1600 C. Some lab-grown diamonds are being subjected to post-growth treatment processes (such as irradiation or annealing, or both) to change their colors (and, in some cases, other gemological properties such as UV fluorescence). Thus, the gemologist is now confronted with the need to recognize faceted synthetic diamonds with colors that are not only “as-grown” (yellow to yellow-brown to brown, blue, green, and colorless), but also result from post-growth treatment processes (yellow, yellow-brown, brown, pink, red, purple, green, or blue-green), as described in Shigley et al. (2004).
           While the information presented in the 1995 chart remains valid, the contents of the updated chart reflect the wider variety of HPHT-grown synthetic diamonds now in the marketplace. However, this new chart is not a comprehensive guide to the identification of as-grown and treated synthetic diamonds; rather, it provides an overview of the common characteristics of them from their natural counterparts.
           Recently, synthetic diamonds suitable for jewelry use have also been produced in small numbers at high temperature but low pressures by the chemical vapor deposition (CVD) process. This material, which is not yet commercially available for jewelry purposes, has very different gemological properties from HPHT-grown samples and, therefore, is not included in this new chart. For further information on CVD-grown material, see Wang et al. (2003) and Martineau et al. (2004).
Color Grading
GIA
Old Terms
A name which use to
call in Europe
Top Wesselton or Fine White
Jager
Exceptional WhiteOrFinest White
River
Fine White
Top Crystal or Commercial White
Top Wesselton
Wesselton
White
Top Crystal
Commercial White
Crystal
Top Silver Cape
Crystal or Top Silver Cape
Top Cape
Silver Cape
Cape
Light Cape
Top Cape or Silver Cape
Low Cape
Very Light Yellow
Cape
Low Cape
Light Yellow
Dark Cape
Fancy
    


SYNTHETIC DIAMOND IDENTIFICATION
The ability to recognize a synthetic diamond first requires an understanding of the kinds of as-grown and treated materials that are now available. Overall production of gem-quality crystals remains very limited to the best of our knowledge, perhaps 12,000 carats per year. Almost all are colored crystals up to about 2 ct (with faceted material up to about 1 ct). It is now possible to produce synthetic diamonds that contain little nitrogen and, as a result, might not be strongly colored. However, growth of type IIa colorless material continues to difficult to achieve in the laboratory, and we do not believe it is available in significant quantities for jewelry purposes. GIA has documented only a few faceted colorless synthetic diamonds obtained from the gem trade during the past decade (see, e.g., Rockwell, 2004).
           In recent years, improvements in growth technology and techniques have resulted in colored synthetic diamond crystals that are larger, have lower impurity contents, and are better quality. This finer quality is evident in the presence of few if any metallic inclusions and flaws, as well as less obvious color zoning in some cases. Nonetheless, lab-created diamonds can still be recognized by a variety of methods. Numerous articles (including the present one) and shorter reports describing these methods, which were published in Gems Gemology over the past 30 years, have been collected together for a special volume that will be made available by GIA in early 2005 (Shigley, in preparation). By reviewing this information, as well as what is presented on this chart, the gemologist will be better prepared to recognize this material. The key identifying features of synthetic diamonds are summarized below.

Crystal Shape and Growth Structure. Natural diamond crystals typically exhibit an octahedral form, with many variations due to growth and/or dissolution (Orlov, 1997, pp., 59-106; Wilks and Wilks, 1994, pp. 108-126). In contrast, synthetic diamonds usually have a cuboctahedral form, cubic, and dodecahedral internal growth sectors. In a vertical orientation, these sectors radiate upwards and outwards from the seed location at the base of the crystal (see Welbourn et al., 1996, p. 162). Diamond crystallization is accompanied by the incorporation of difficult amounts of impurities in these sectors-thus leading to a segregation of these impurities between sectors. Differential incorporation of impurities gives rise to the distinctive zoning of color, graining, and luminescence seen in many synthetic (as compared to natural) diamonds. When present, boundaries between adjacent color zones are usually sharp and planar they also may intersect to form angular patterns. Adjacent zones of very different color. For example, certain lab-grown green samples now being sold by Chatham Created Gems exhibit both yellow and blue growth sectors when examined with a microscope (see Shigley et al., 2004). Post-growth color treatment processes do not obscure or remove these distinctive visual features, although it may be possible to lesson the visibility of the color zoning during growth (especially if one growth sector predominates within the crystal, while other sectors of differing color are smaller and thus less obvious).
           Careful examination using a gemological microscope and different lighting technique is the best way to see this growth sector-related color zoning in lab-grown diamonds. Immersion of the sample in a liquid (even water) for better observation is also helpful. Such zoning should be evident as well when the sample is examined with a standard UV fluorescence unit or the DTC DiamondView. Depending on the viewing orientation, the zoning can display two-, three-, or four-fold patterns related to the diamond’s cubic crystal symmetry. In most cases, the table facet of a polished sample is oriented approximately parallel to the cube face of the original crystal for maximum weight retention during faceting. Therefore, it is often best to look for any four-fold color or fluorescence zoning pattern by observing through the table or crown facets-or, alternatively, nearly parallel to the girdle facets-while rotating the sample. The key is to examine a sample in several orientations to by distinct planar boundaries.

Inclusions, Graining, and “Strain” Patterns. Unless they are prevented from forming during growth, or are physically removed during faceting, metallic inclusions are a common feature in many polished synthetic diamonds. They may be rounded, elongate, or irregular in shape, and will appear opaque in transmitted light and dark gray-to-black (sometimes with a metallic luster) in reflected light. They may occur singly or in groups, and can vary in size. In some cases, their large size makes them virtually eye-visible whereas in other instances, they are so tiny as to be described as “pinpoint” inclusions, which are often seen in diffuse, cloud-like arrangement (Note that although some of these pinpoint inclusions may be metallic, others may represent different phases formed during synthesis.) Some of these inclusions may even be invisible with the magnification of a standard gemological microscope. Because the flux inclusions often contain iron, they can result in the synthetic diamond being attracted to a magnet.
           Natural diamonds may display linear, crosshatched, or irregular (“mosaic”) internal graining patterns (Kane, 1980). In synthetic diamonds, internal graining in linear or intersecting geometric patterns appears to be the result of slight differences in refractive index between adjacent growth sectors, or between successive parallel “layers” of material beneath the crystal faces. It is best seen along the boundaries between sectors, or in planes that parallel the outer shape of the original crystal. Since the cuboctahedral crystals are often faceted in square or rectangular shapes for weigh retention , one good place to check for graining in faceted samples is near the corners of the table facet (and adjacent crown facets) with magnification (a fiber-optic illuminator can be quite helpful).
           Most natural diamond exhibit anomalous double reflaction (ADR) in banded, cross-hatched, or mottled patterns with bright interference colors (when observed through crossed polarizing filters; see Orlov, 1977, pp. 109-116). In comparison, our weaker, cross-like “strain” patterns with subdued interference colors (black or gray).

Luminescence. Given the wide variety of synthetic diamonds now available, their reactions to long and short-wave UV radiation can differ greatly in terms of fluorescence intensity, color, distribution pattern, and phosphorescence. While it has been widely reported that most lab-grown samples display stronger fluorescence to short-wave UV than to long-wave, the opposite reaction has also been observed (as well as the same intensity reaction to both UV lamps), and some samples are inert to both UV excitations. To check for weak UV fluorescence reactions, it is best to observe the sample while in a darkened room, after the eyes have had time to adjust to low light levels. In more recent rears, we have noticed an increasing number of synthetic diamonds that display only weak UV fluorescence, or no fluorescence reaction at all.
           As mentioned, fluorescence colors can also vary, but typically they range from green to blue to yellow to orange or orange-red. More importantly, however, this fluorescence is often unevenly distributed, so that some portions of the sample fluoresce whereas others do not (or they fluoresce with different colors). This uneven distribution is again a reflection of the arrangement of internal growth sectors with their differing impurity contents, so there is a direct spatial relationship between color, graining, and UV fluorescence patterns. In the most obvious cases, this uneven fluorescence is seen as a square and/or cross-shaped geometric pattern. Again, the orientation of the faceted shape with respect to the original crystal will influence how color, graining, and fluorescence patterns appear, so it is important to examine a sample in several orientations.
           Similar fluorescence patterns in synthetic diamonds can be observed using the cathodoluminescence technique (where the sample is exposed to abeam of electrons while being held in a vacuum chamber). The DTC DiamondView, where fluorescence reactions are excited by exposure of the sample to UV radiation with wavelengths shorter than 230 nm, also provides an provides an excellent tool for viewing surface-related fluorescence and phosphorescence patterns in a sample at different orientations (see Welbourn et al., 1996).
           Colorless synthetic diamonds, and any colored samples that contain boron as an impurity, frequently display persistent greenish or yellowish phosphorescence is a phenomenon that decreases in intensity over time, it is again important to check for this kind of luminescence by viewing the samples in a darkened room. A good technique is to close one’s eyes, and then open them at the same time the UV lamp is turned off. Blue (and some near colorless) synthetic conductivity and, interestingly, will often display visible electroluminescence in the form of momentary tiny flashes of white to bluish white light when the samples are touched by the conductometer probe.

Chemical and Spectroscopic Analysis. Nondestructive methods of chemical analysis provide another rapid means of identifying synthetic diamonds by detecting flux metals (Ni, Co, and Fe) that are used in diamond growth. Particularly useful in recognizing lab-created diamonds, especially those that lack distinctive visual features, are several spectroscopy techniques that are found today in many gemological laboratories. Because diamond is relatively transparent from the infrared through the visible and ultraviolet regions of the electromagnetic spectrum, numerous absorption and emission features can be detected by these techniques (Zaitsev, 2001, lists the spectral features individually along with a brief description of what is known about them). Specific bands caused by the presence of transition metals are valuable for detecting either as grown or treated synthetic diamonds by visible spectroscopy (for example, those at 494, 658, and 732 nm, as well as several others, which are all due to nickel; see again Zaitsev, 2001). Caution must be exercised, however, as we now know that some natural diamonds contain small amounts of nickel (see, e.g., Chalain, 2003; Lang et al., 2004; Hainchwang and Notari, 2004). Photoluminescence (PL) spectroscopy is increasingly important for gem laboratories, since many of the optical centers in diamond have associated sharp PL bands that are useful for identification purposes. The interested reader is referred to articles cited in the reference list for examples of the application of these and other spectroscopy methods to diamond characterization (see, e.g., Lawson et al., 1996; Collins, 2000, 2001; Zaitsev, 200, 2001; Yelisseyev et al., 2002) . Additional analytical techniques for detecting synthetic diamonds may become useful in the future.