Christopher M. Breeding

The "Type" Classification System of Diamonds and Its Importance in Gemology

mond growth, color (e.g., figure 1), and response to laboratory treatments. With the increasing availability of treated and synthetic diamonds in the marketplace, gemologists will benefit from a more complete understanding of diamond type and of the value this information holds for diamond identification. Considerable scientific work has been done on this topic, although citing every reference is beyond the scope of this article (see, e.g., Robertson et al., 1934, 1936; and Kaiser and Bond, 1959). Brief gemological discussions of diamond types appeared in Shigley et al. (1986), Fritsch and Scarratt (1992), and Smith et al. (2000), and more-detailed descriptions were given in Wilks and Wilks (1991) and Collins (2001). Nevertheless, repeated inquiries received at GIA indicate that many practicing gemologists do not have a clear understanding of the basics of diamond type. This article offers a readily accessible, gemology-specific guide to diamond type and related THE “TYPE” CLASSIFICATION SYSTEM OF DIAMONDS AND ITS IMPORTANCE IN GEMOLOGY

TREATED-COLOR PINK-TO-RED DIAMONDS FROM LUCENT DIAMONDS INC.

Lucent Diamonds has developed a new treatment process for natural type Ia diamonds that produces colors ranging from pink-purple through red to orangy brown, using a multi-step process that involves HPHT annealing, irradiation, and low-pressure annealing at relatively lower temperatures. Those stones that achieve a predominant pink-to-red or purple color are marketed as “Imperial Red Diamonds.” Gemological properties and characteristic spectra are presented for 41 diamonds, representing the range of colors produced thus far. These treated-color natural diamonds can be readily identified by internal graphitization and surface etching seen with magnification, distinctive color zoning, and reactions to long- and short-wave UV radiation. The color is caused primarily by the absorption of the (N-V) - center, with further influence from the (N-V) 0 , H3, H4, and N3 centers. Other characteristic infrared and UV-visible absorption features include the H1a, H1b, H1c, 6170 cm -1 , and, frequently, 594 nm bands. This type of defect combination

Copper-bearing (Paraíba-type) tourmaline from Mozambique

Copper-bearing tourmaline from Mozambique was first recovered in 2001, but its Cu content was not recognized until 2003, and it was not widely sold with its Mozambique origin disclosed until 2005. It has been mined from alluvial deposits in an approximately 3 km2 area near Mavuco in the eastern portion of the Alto Ligonha pegmatite district. Most of the production has come from artisanal mining, with hand tools used to remove up to 5 m of overburden to reach the tourmaline-bearing layer. The stones exhibit a wide range of colors, typically pink to purple, violet to blue, and blue to green or yellowish green. Heat treatment of all but the green to yellowish green stones typically produces Paraiba-like blue-to-green hues by reducing absorption at ∼520 nm caused by the presence of Mn3+. The gemological properties are typical for Cu-bearing tourmaline (including material from Brazil and Nigeria); the most common inclusions consist of partially healed fractures and elongate hollow tubes. With the exception of some green to yellow-green stones, the tourmalines examined have relatively low Cu contents and very low amounts of Fe and Ti. Mechanized mining is expected to increase production from this region in the near future.

Fluorescence Spectra of Colored Diamonds Using A Rapid, Mobile Spectrometer

have been chronicled in natural diamonds using UVVis absorption, cathodoluminescence, and photoluminescence spectroscopy (see, e.g., Zaitsev, 2001), spectral data for fluorescence and phosphorescence are limited in the gemological literature, since luminescence is typically described by visual observations (again, see Fritsch and Waychunas, 1994). However, visual assessment of fluorescence and phosphorescence tells only part of the story. The color discerned by the unaided eye may represent a combination of two or more wavelength regions. For example, Anderson (1960) asserted that although most fluorescing diamonds appear to luminesce blue, a yellow or green component may be present but masked by the stronger blue emission. He used color filters and a spectroscope to try to identify some of the relevant FLUORESCENCE SPECTRA OF COLORED DIAMONDS USING A RAPID, MOBILE SPECTROMETER

Lab-Grown Colored Diamonds From Chatham Created Gems

developments in recent years has been the commercial availability of jewelry-quality synthetic diamonds. What for almost three decades was primarily an industrial or research product is now becoming a commodity in the gem and jewelry marketplace. In addition to the products being offered by such companies as the Gemesis Corp. and Lucent Diamonds, Chatham Created Gems of San Francisco, California, has introduced a line of synthetic diamonds from a new source (figure 1). This article presents results of our examination of a large group of these high pressure/high temperature (HPHT) laboratory-grown diamonds in yellow, blue, green, and pink colors showing a full range of saturation, from weak to strong. Our examination indicates that most of the yellows and blues represent “as-grown” colors (i.e., those produced by nitrogen and boron impurities during diamond crystallization), while the greens and pinks are the result of either growth or growth plus post-growth treatment processes (i.e., irradiation, with or without subsequent heating). A single manufacturer is supplying Chatham Created Gems with approximately 500 carats of synthetic diamond crystals per month, with future increases in production planned (T. Chatham, pers. comm., 2004). The material is faceted in China into cut goods that range from a few points (melee) to as large as 2 ct. Chatham Created Gems is the sole distributor of this material for jewelry purposes. Previous gemological reports on synthetic diamonds produced in Russia and sold by Mr. Chatham (see, e.g., Scarratt et al., 1996) may not be applicable to the new HPHT-grown material described here, which is grown in Asia with a non-BARS press. This article provides information on material from all four color categories of this new product, including descriptions of green and pink synthetic diamonds, which have not been reported on extensively in the gemological literature. Most of the green samples display this color because they contain both blue and yellow growth sectors. Some of this new material displays hues and weaker saturations that more closely resemble natural diamonds

The Effects of Heat Treatment on Zircon Inclusions in Madagascar Sapphires

GEMS & GEMOLOGY SUMMER 2006 Zircon inclusions in sapphires from Madagascar were studied to investigate the effects of heat treatment on their gemological and spectroscopic features. Progressive decomposition of zircon and chemical reactions between zircon and the host sapphire occurred at temperatures between 1400°C and 1850°C. In unheated sapphires, transparent zircon inclusions displayed euhedral slightly elongated forms and clear interfaces with their corundum host. Most were confined within the host under relatively high pressures (up to 27 kbar), and showed evidence of natural radiation-related damage (metamictization). Subsolidus reactions (i.e., the decomposition of zircon into its component oxides without melting) of some zircon inclusions started at temperatures as low as 1400°C, as evidenced by the formation of baddeleyite (ZrO2) and a SiO2-rich phase. Differences in the degree of preexisting radiation damage are the most likely cause for the decomposition reactions at such relatively low temperatures. Melting of zircon and dissolution of the surrounding sapphire occurred in all samples at 1600°C and above. This resulted in the formation of both baddeleyite and a quenched glass rich in Al2O3 and SiO2. From these data and observations, a systematic sequence of both modification and destruction of zircon inclusions with increasing temperature was compiled. This zircon alteration sequence may be used (1) as a gemological aid in determining whether a zircon-bearing ruby/sapphire has been heated, and (2) to provide an estimate of the heating temperature.

Optical Defects in Diamond: A Quick Reference Chart

D the commercial value of natural-color diamonds, distinguishing them from treated diamonds remains a significant identification challenge. While some diagnostic visual features exist (inclusions, color or growth zoning, and absorption bands seen with a spectroscope), the separation of natural from synthetic or treated diamonds is not always possible using standard gemological methods. In such cases, advanced spectroscopic analysis at a professional gem-testing laboratory is required. Imaging of luminescence distribution patterns is also a helpful tool for recognizing synthetic diamonds (Martineau et al., 2004; Shigley et al., 2004). In a laboratory setting, the identification of diamonds is based mainly on the detection of tiny imperfections in the atomic lattice. These “defect centers” may include foreign impurity atoms (typically nitrogen, and occasionally boron or hydrogen); carbon atom vacancies in the lattice (either single or clusters of neighboring vacancies); carbon atoms positioned in between normal lattice locations (interstitials); and dislocations where planes of carbon atoms are offset from one another due to plastic deformation. Not all of these lattice imperfections create spectroscopic features, but several do so by allowing the

An Updated Chart on The Characteristics of HPHT-Grown Synthetic Diamonds

GEMS & GEMOLOGY WINTER 2004 303 lmost 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 jewelry applications (figure 1), with advertisements for synthetic diamonds seen occasionally in trade publications and other industry media. Recent inquiries to three distributors in the U.S.— Chatham Created Gems of San Francisco, California; Gemesis 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 recA AN UPDATED CHART ON THE CHARACTERISTICS OF HPHT-GROWN SYNTHETIC DIAMONDS

Developments in Gemstone Analysis Techniques and Instrumentation During the 2000s

The first decade of the 2000s continued the trend of using more powerful analytical instruments to solve gem identification problems. Advances in gem treatment and synthesis technology, and the discovery of new gem sources, led to urgent needs in gem identification. These, in turn, led to the adaptation of newer scientific instruments to gemology. The past decade witnessed the widespread use of chemical microanalysis techniques such as LA-ICP-MS and LIBS, luminescence spectroscopy (particularly photoluminescence), real-time fluorescence and X-ray imaging, and portable spectrometers, as well as the introduction of nanoscale analysis. Innovations in laser mapping and computer modeling of diamond rough and faceted stone appearance changed the way gemstones are cut and the manner in which they are graded by gem laboratories.

An Introduction to Photoluminescence Spectroscopy for Diamond and Its Applications in Gemology

Photoluminescence (PL) spectroscopy is frequently mentioned in the gemological literature, but its relevance to the wider trade audience is rarely discussed. Due to the possibility of an undisclosed treatment or a synthetic origin, all type II diamonds (both colorless and fancy-color) and colorless type IaB diamonds submitted to gemological laboratories should ideally be tested using PL spectroscopy. Although the proportion of samples that require this testing is small, the failure to properly identify treated and synthetic diamonds could destabilize the diamond industry. This article seeks to clarify the underlying physics and methodology of this important tool for gemologists.