James E. Shigley

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

Gem-Quality Synthetic Diamonds Grown by a Chemical Vapor Deposition (CVD) Method

chusetts, has succeeded in growing facetable, single-crystal type IIa synthetic diamonds using a patented chemical vapor deposition (CVD) technique (Linares and Doering, 1999, 2003). For characterization of this material, the company has provided a number of brown-to-gray and near-colorless gem-quality crystals and faceted samples, which represent what is intended to become a commercial product for use in high-technology applications as well as for jewelry purposes (figure 1). Because of the growth conditions and mechanisms used, the gemological properties of these CVD-produced synthetic diamonds differ from those of both natural diamonds and synthetic diamonds grown at high pressures and temperatures. For the same reasons, the brown coloration of CVD synthetic diamonds may not react in the same way, or as efficiently, as most natural type IIa brown diamonds, which can be decolorized at high pressure and high temperature (see box A). Preliminary notes on GIA’s examination of some of these Apollo samples were published by Wang et al. (2003) and appeared in the August 8, 2003 issue of the GIA Insider (GIA’s electronic newsletter: http://www.gia.edu/newsroom/issue/2798/1842/ insider_newsletter_details.cfm#3). Spectroscopic analysis of an Apollo CVD synthetic diamond also was performed recently by other researchers (Deljanin et al., 2003). The purpose of the present article is to provide a more complete description of this material and its identifying features.

The Gemological Properties of the De Beers Gem-Quality Synthetic Diamonds

Gem-quali ty synthetic diamond crystals weighing up to 11 ct have been grown in limited numbers at the De Beers Diamond Research Laboratory since the 1970s. These crystals have been produced strictly on an experimental basis and are not commercially available. Examination of a group of 14 brown1s17 yellow, yellow, and greenish yellow synthetic diamonds reveals distinctive gemological properties: uneven color distribution, geometric graining patterns, metallic inclusions, and, i n most cases, fluorescence t o shortwave but not to long-wave U.V radiation.

The Gemological Properties of the Sumitomo Gem-Quality Synthetic Yellow Diamonds

The distinctive gemological properties of the gem-quality synthetic yellow diamonds grown by Sumitomo Electric Industries are described. These synthetic diamonds, produced on a commercial basis, are grown as deep yellow single crystals i n sizes up to 2 ct. The material i s currently marketed for industrial applications only, in pieces up to about 0.40 ct. The synthetic diamonds can be distinguished by their ultraviolet fluorescence (inert to long-wave; greenish yellow or yellow to short-wave); their unusual graining, veining, and color zonation under magnification; and the absence of distinct absorption bands in their spectra.

Separating Natural and Synthetic Rubies on the Basis of Trace-Element Chemistry

GEMS & GEMOLOGY Summer 1998 orrect gem identification is crucial to the gem and jewelry trade. However, accurate information on a gem’s origin rarely accompanies a stone from the mine, or follows a synthetic through the trade after it leaves its place of manufacture. Today, natural and synthetic rubies from a variety of sources are seen routinely (figure 1). Usually, careful visual observation and measurement of gemological properties are sufficient to make important distinctions (Schmetzer, 1986a; Hughes, 1997). In some cases, however, traditional gemological methods are not adequate; this is particularly true of rubies that are free of internal characteristics or that contain inclusions and growth features that are ambiguous as to their origin (Hänni, 1993; Smith and Bosshart, 1993; Smith, 1996). The consequences of a misidentification can be in the tens of thousands, and even hundreds of thousands, of dollars. Ruby is a gem variety of corundum (Al2O3) that is colored red by trivalent chromium (Cr3+). Besides Cr, most rubies contain other elements in trace amounts that were incorporated during their growth, whether in nature or in the laboratory. For the purpose of this article, we consider trace elements to be those elements other than aluminum, oxygen, and chromium. These trace elements (such as vanadium [V] and iron [Fe]) substitute for Al3+ in the corundum crystal structure, or they may be present as various mineral inclusions (such as zirconium [Zr] in zircon) or as constituents in fractures. The particular assemblage of trace elements (i.e., which ones are present and their concentrations) provides a distinctive chemical signature for many gem materials. Since the trade places little emphasis on establishing the manufacturer of synthetic products, this article will focus on how trace-element chemistry, as determined by EDXRF, can be used for the basic identification of natural versus synthetic rubies. It will also explore how EDXRF can SEPARATING NATURAL AND SYNTHETIC RUBIES ON THE BASIS OF TRACE-ELEMENT CHEMISTRY

Gem-Quality Cuprian-Elbaite Tourmalines from São José Da Batalha, Paraíba, Brazil

Unusually vivid tourmalines from the state of Paraiba, in northeastern Brazil, have attracted great interest since they first appeared on the international gem market in 1989. This article describes what is known of the locality at this time, but focuses on the most striking characteristic of these gem tourmalines: the unusual colors in which they occur. Quantitative chemical analyses revealed that these elbaite tourmalines contain surprisingly high concentrations of copper, up to 1.92 wt.% Cu (or 2.38 wt.% CuO). Their colors are due to Cu^(2+) or a combination of Cu^(2+), Mn^(3+), and other causes. Some colors can be produced by heat treatment, but most also occur naturally.

Identification of HPHT-Treated Yellow to Green Diamonds

128 Yellow to Green HPHT Diamonds GEMS & GEMOLOGY Summer 2000 everal companies are now treating brown diamonds with high pressure and high temperature (HPHT) to transform their color to greenish yellow, yellowish green, yellow, or brownish yellow (figure 1). These include the General Electric (GE) Company, Novatek, and unidentified organizations in both Russia and Sweden. Both GE and Novatek are concentrating their efforts on producing colors with an obvious green component (Templeman, 2000; Federman, 2000; Anthony et al., 2000). In contrast, the Swedish manufacturer apparently is trying to produce colors similar to those of natural yellow diamonds (J. Menzies, pers. comm., 2000). Moses and Reinitz (1999) briefly described some of the treated diamonds from these various manufacturers. The present article reports the gemological and spectroscopic properties of a large number of diamonds treated in this fashion, and discusses how the gemological properties compare to those of the rarest and most desirable of the stones that they resemble: natural-color greenish yellow to yellow-green diamonds.

MODELING THE APPEARANCE OF THE ROUND BRILLIANT CUT DIAMOND: AN ANALYSIS OF BRILLIANCE

Of the “four C’s,”cut has historically been the most complex to understand and assess. This article presents a three-dimensional mathematical model to study the interaction of light with a fully faceted, colorless, symmetrical roundbrilliant-cut diamond. With this model, one can analyze how various appearance factors (brilliance, fire, and scintillation) depend on proportions. The model generates images and a numerical measurement of the optical efficiency of the round brilliant—called weighted light return (WLR)—which approximates overall brilliance. This article examines how WLR values change with variations in cut proportions, in particular crown angle, pavilion angle, and table size. The results of this study suggest that there are many combinations of proportions with equal or higher WLR than “Ideal” cuts. In addition, they do not support analyzing cut by examining each proportion parameter independently. However, because brilliance is just one aspect of the appearance of a faceted diamond, ongoing research will investigate the added effects of fire and scintillation.

A Contribution to Understanding the Effect of Blue Fluorescence on the Appearance of Diamonds

GEMS & GEMOLOGY Winter 1997 any factors influence the color appearance of colorless to faint yellow diamonds. Typically, such diamonds are quality graded for the absence of color according to the D-to-Z scale developed by the Gemological Institute of America in the 1940s (Shipley and Liddicoat, 1941). By color appearance, however, we mean the overall look of a polished diamond’s color that results from a combination of factors such as bodycolor, shape, size, cutting proportions, and the position and lighting in which it is viewed. When exposed to invisible ultraviolet (UV) radiation, some diamonds emit visible light, which is termed fluorescence (figure 1). This UV fluorescence arises from submicroscopic structures in diamonds. Various colors of fluorescence in diamond are known, but blue is by far the most common. The response of a diamond to the concentrated radiation of an ultraviolet lamp is mentioned as an identifying characteristic (rather than a grading factor) on quality-grading reports issued by most gem-testing laboratories. Other light sources—such as sunlight or fluorescent tubes—also contain varying amounts of UV radiation. Although there have been instances where the color and strength of the fluorescence seen in diamonds observed in these other light sources are also believed to influence color appearance, in recent years the fluorescence noted on grading reports has been singled out by many in the diamond trade and applied across the board as a marker for pricing distinctions. Generally, these distinctions are applied in the direction of lower offering prices for colorless and near-colorless diamonds that exhibit fluorescence to a UV lamp (Manny Gordon, pers. comm., 1997). Other trade members contend that the overall color appearance of a diamond typically is not adversely affected by this property (William Goldberg, pers. comm., 1997); many even say that blue fluorescence enhances color appearance. A CONTRIBUTION TO UNDERSTANDING THE EFFECT OF BLUE FLUORESCENCE ON THE APPEARANCE OF DIAMONDS

Gem-Quality Red Beryl from the Wah Wah Mountains, Utah

A detailed investigation of the gem-quality red beryl from the southern Wah Wah Mountains, Utah, has confirn~ed the unique mineralogical and gemological character of this material. A t the Violet Claims, red beryl is found associated with minor bixbyite in a volcanic rhyolite host rock. Analytical data gathered on the red beryl indicate relatively high contents of the minor or trace elements Mn, Ti, Zn, Sn, Li, Nb, Sc, Zr, Ga, Cs, Rb, B, and Pb, which are generally low or absent in other gem beryls. Measured refractive indices (1.564 -1.574), specific gravity (2.66 -2.70), and unit-cell parameters (a= 9.222 A. c= 9.186 A) of the red beryl are distinct from most other beryls. The red beryl is thought to have crystallized along fractures, i n cavities, or within the host rhyolite from a high-temperalure gas or vapor phase released during the latter stages of cooling and crystallization of the rhyolite magma.