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An Updated Chart on the Characteristics of Hpht-Grown Synthetic Diamonds

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An Updated Chart on the Characteristics of Hpht-Grown Synthetic Diamonds AN UPDATED CHART ON THE CHARACTERISTICS OF HPHT-GROWN SYNTHETIC DIAMONDS James E. Shigley, Christopher M. Breeding, and Andy Hsi-Tien Shen A new chart, supplementing the one published in the Winter 1995 issue of Gems & Gemology, summarizes the features of both as-grown (“non-modified”) and treated (“modified”) synthetic diamonds currently in the gem market (that is, those grown by the high pressure/high temperature technique). It includes photographs of visual features, information about visible-range absorption spectra, and illustrations of growth-structure patterns as revealed by ultraviolet fluorescence imag- ing. The chart is designed to help gemologists recognize the greater variety of laboratory-created diamonds that might be encountered today. lmost a decade ago, Shigley et al. (1995) pub- fornia; Gemesis Corp. of Sarasota, Florida; and lished a comprehensive chart to illustrate Lucent Diamonds Inc. of Lakewood, Colorado— A the distinctive characteristics of yellow, col- indicate that their combined production of crystals orless, and blue natural and synthetic diamonds. The is on the order of 1,000 carats per month (mainly yel- accompanying article reviewed synthetic diamond low colors), a quantity that does not meet their cus- production at the time, and discussed how the infor- tomer demand. mation presented on the chart was acquired and The synthetic diamonds currently in the gem organized. It also included a box that provided a market are grown at high pressure and high temper- “practical guide for separating natural from synthet- ature (HPHT) conditions by the temperature-gradi- ic diamonds.” The chart was distributed to all Gems ent technique using several kinds of high-pressure & Gemology subscribers, and a laminated version equipment (belt, tetrahedral, cubic, and octahedral was subsequently made available for purchase. presses as well as BARS apparatuses), and one or Since that time, and especially within the past more transition metals (such as Ni, Co, and Fe) as a several years, the situation of synthetic diamonds in flux solvent/catalyst. Typical growth temperatures the jewelry marketplace has become more compli- are 1350–1600°C. Some lab-grown diamonds are cated. Lab-created colored diamonds are now being being subjected to post-growth treatment processes produced in several countries (including Russia, the (such as irradiation or annealing, or both) to change Ukraine, Japan, the U.S., and perhaps China and their colors (and, in some cases, other gemological elsewhere), although the quantities continue to be properties such as UV fluorescence). Thus, the very limited. And today they are being sold specifi- gemologist is now confronted with the need to rec- cally for jewelry applications (figure 1), with adver- tisements for synthetic diamonds seen occasionally in trade publications and other industry media. See end of article for About the Authors and Acknowledgments. Recent inquiries to three distributors in the U.S.— GEMS & GEMOLOGY, Vol. 40, No. 4, pp. 303–313. Chatham Created Gems of San Francisco, Cali- © 2004 Gemological Institute of America HPHT-GROWN SYNTHETIC DIAMOND CHART GEMS & GEMOLOGY WINTER 2004 303 Figure 1. HPHT-grown Synthetic diamonds are now available in the gem and jewelry market- place, as is evident from this attractive 1.00–1.25 ct synthetic yellow dia- mond jewelry provided by Gemesis Corp. (the colorless diamonds are natural) and the (each under 1 ct) loose synthet- ic diamonds from Lucent Diamonds and Chatham Created Gems. Composite photo: jewelry images courtesy of Gemesis Corp.; loose diamond photos by Harold & Erica Van Pelt. CONTENTS OF THE NEW CHART ognize faceted synthetic diamonds with colors that are not only “as-grown” (yellow to yellow-brown to The new chart is organized differently from the one brown, blue, green, and colorless), but also result published in 1995 (figure 2). Given the greater from post-growth treatment processes (yellow, yel- amount of information and broader variety of mate- low-brown, brown, pink, red, purple, green, or blue- rial available, the present chart focuses entirely on green), as described in Shigley et al. (2004). synthetic diamonds and does not include entries for While the information presented in the 1995 natural diamonds. (The reader is directed to the ref- chart remains valid, the contents of the updated erences cited in Shigley et al., 1995—and informa- chart reflect the wider variety of HPHT-grown syn- tion provided on that chart—for information on nat- thetic diamonds now in the marketplace1. However, ural as well as early synthetic diamonds. Also, dur- this new chart is not a comprehensive guide to the ing the past decade, several books and articles have identification of as-grown and treated synthetic dia- described many of the features of both natural and monds; rather, it provides an overview of the com- lab-created diamonds; these are listed in the mon characteristics of these materials, which can Additional Reading section at the end of this article.) be helpful in separating them from their natural Regardless of their color, synthetic diamonds counterparts. grown by the HPHT technique from a molten Recently, synthetic diamonds suitable for jewel- metal flux have some common characteristics as a ry use have also been produced in small numbers at result of their growth conditions. These include high temperatures but low pressures by the chemi- their cuboctahedral crystal shape (figure 3), growth cal vapor deposition (CVD) process. This material, features (such as surface markings, color zoning, which is not yet commercially available for jewelry and graining), and metallic inclusions. Repre- purposes, has very different gemological properties sentative photos and photomicrographs illustrating from HPHT-grown samples and, therefore, is not these three types of characteristics are grouped included in this new chart. For further information together across the upper portion of the new chart on CVD-grown material, see Wang et al. (2003) and as a way to emphasize their common occurrence in Martineau et al. (2004). HPHT-grown synthetic diamonds from all current manufacturers. 1Reports in the scientific literature indicate that as-grown synthetic dia- The lower portion of the chart is divided into monds with a green color can also be produced when growth occurs two sections—one for information on synthetic dia- from a nickel solvent-catalyst along with a component in the flux that monds with as-grown (or “non-modified”) colors, actively combines with nitrogen (i.e., a nitrogen “getter”; see Chrenko and Strong, 1975; Kanda, 1999). To the best of our knowledge, this and the other for those with treated (or “modified”) kind of synthetic diamond is not presently available in the market. colors (figure 4). For the latter, the entries are divid- 304 HPHT-GROWN SYNTHETIC DIAMOND CHART GEMS & GEMOLOGY WINTER 2004 Figure 2. The chart pub- lished by Shigley et al. in 1995 included informa- tion on natural diamonds as well as on the kinds of HPHT-grown synthetic diamonds available at that time. ed into the following four categories: (1) HPHT The information presented in the chart is based annealing, (2 and 3) irradiation plus annealing at on data collected at GIA over the past 25 years on two different temperatures, and (4) irradiation only. approximately 500 synthetic diamonds from all Organizing information in this way is not meant to known sources of production. The photos and pho- imply that the distinction of untreated and treated tomicrographs were selected to illustrate those visu- colors in synthetic diamonds is important. Rather, al features of lab-grown diamonds useful for identifi- it is designed to help the gemologist who must test cation purposes. an unknown sample with a color that might not at We do not indicate the manufacturer or distributor first be considered typical of synthetic diamonds. In of the synthetic diamonds illustrated on the chart for addition, in some cases—such as yellow or green— two reasons. First, we know that once a synthetic dia- the color may be either as-grown or treated. mond is sold in the trade, such information may no In this lower portion of the chart, the entries are longer be available (unless a distinctive marking visi- presented in a column format by color and diamond ble with magnification is placed on the girdle surface type (a grouping of diamonds into one of several cat- egories based on their physical and spectral proper- ties; see, e.g., Fritsch and Scarratt, 1992; Wilks and Figure 3. HPHT-grown synthetic diamond crystals Wilks, 1994, pp. 62–82). Presenting information in are usually cuboctahedral in shape, as illustrated this way provides a basis for better understanding by these colored synthetic diamonds from the properties of the samples in each category. The Chatham Created Gems, which weigh between 0.44 and 1.74 ct. Photo by Maha Tannous. visual features summarized in these two sections are supplemented by representative visible-range absorption spectra, as well as by ultraviolet fluores- cence images of growth structure obtained with the Diamond Trading Company (DTC) DiamondView instrument (Welbourn et al., 1996). Such data are increasingly important to confirm the identity of some synthetic diamonds. Information obtained by nondestructive chemical analyses for transition metals (such as Ni and Fe), as well as by other spec- troscopic (infrared and photoluminescence), cathodoluminescence, and analytical techniques available in the larger gemological laboratories, may also be useful for synthetic diamond recognition. HPHT-GROWN SYNTHETIC DIAMOND CHART GEMS & GEMOLOGY WINTER 2004 305 Figure 4. Synthetic diamonds currently sold for jewelry purposes display a range of as-grown and treated colors. The as-grown yellow crystal (2.43 ct) and three yellow faceted samples (0.28–0.84 ct) shown on the left represent the most common kind of synthetic diamond produced today. The colors of the green-to-blue faceted samples (0.20–0.40 ct) shown in the center also were produced during growth.
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