Ilene M. Reinitz

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

Modeling the Appearance of the Round Brilliant Cut Diamond: An Analysis of Fire, and More About Brilliance

174 mODeLING FIRe GemS & GemOLOGy FALL 2001 al system of the overall pattern of light shown by the diamond (figure 1). Traditionally, the appearance of the round brilliant diamond has been described using three aspects: brilliance, fire, and scintillation. A method that scientists use to address a complicated problem is: (1) break it into simpler aspects, examining each aspect separately; and then (2) make sure that solutions for each small piece of the problem also hold true for the larger problem as a whole. We have applied this approach to our study of polished diamond appearance by examining each appearance aspect separately. In our report on the first of these (Hemphill et al., 1998), we used a mathematical expression for brilliance, called weighted light return or WLR, which we developed from the definition of brilliance given in the GIA By Ilene m. Reinitz, mary L. Johnson, T. Scott Hemphill, Al m. Gilbertson,

Gemological Investigation of a New Type of Russian Hydrothermal Synthetic Emerald

Tairus, in Novosibirsk, has produced yet another new type ofRussian hydrothermal synthetic emerald, now being marketed in Bangkok. Examination of eight fac eted samples revealed that, with the exception of certain characteristic inclusions, the basic gemological properties shown by this new synthetic are essentially the same as those encountered in other hydrothermally grown synthetic emeralds and some natural emeralds. If the characteristic inclusions are not present, distinctive spectral characteristics in both the midand near-infrared regions ofthe spectrum will serve to separate these synthetic emeralds from their natural counterparts.

AN OSCILLATING VISIBLE LIGHT OPTICAL CENTER IN SOME NATURAL GREEN TO YELLOW DIAMONDS

Abstract Visible spectra taken at 72 K of a group of ten green-to-yellow-colored natural gem-quality diamonds show more than 30 sharp absorption bands between 13140 ∗ and 18420 cm −1 (1.63 and 2.28 eV), suggesting one or two defects that oscillate to visible light, a phenomenon not previously reported for diamond. Preliminary analysis of these spectra shows two oscillations, one dominating the lower energy end of this range with an energy spacing of 0.021 eV, and another appearing at the higher energy end of the range with a spacing of 0.025 eV. All samples show weak to moderate strength infrared absorptions typical of hydrogen, and of single substitutional nitrogen. The green color in nine of these diamonds is caused by the broad overlap at the base of these many absorption bands, not by a GR1 center, and thus forms a new cause of naturally occurring green color in diamond. Similar “many-banded” features have been observed by others in the low temperature cathodoluminescence spectra of diamond and in the low temperature luminescence spectra of minerals containing a molecular ion; perhaps the hydrogen or nitrogen (or both) play a role in causing these oscillations.

Observation of etch channels in several natural diamonds

Abstract We report the characteristics of dissolution etch channels (open tubes), with various forms ranging from parallel lines to irregular ribbon or worm-like shapes, in seven natural gem-quality diamonds. These channels have openings at the surface with rhombic or modified rhombic shapes, and internally, they often terminate at solid inclusions. They appear to originate either from the outcrop of a bundle of dislocations, or along dislocation dipoles elongated along the 〈110〉 direction. As the dissolution process proceeds, the penetration direction of an etch channel may change as a result of the interaction with other growth defects. Many of the etch channels we observed exhibit highly irregular forms. Possible relationships between the etch channels and lattice defects are discussed.

Gia's Symmetry Grading Boundaries For Round Brilliant Cut Diamonds

GEMS & GEMOLOGY WINTER 2011 ations during planning and cutting. Likewise, makers of non-contact optical scanners have been interested in guidelines for how measurable symmetry parameters affect the GIA symmetry grade. The grade boundaries presented here offer a substantive estimate of the symmetry grade for any round brilliant cut diamond. In GIA’s laboratory, polished diamonds are measured with a non-contact optical scanner early in the grading process. Later, polish and symmetry are evaluated visually at 10× magnification, using a standard procedure. As described in Gillen et al. (2005), specific parameterand facet-related features are considered in grading symmetry. This article presents numerical grade limits for 10 important symmetry parameters that can be measured accurately enough to support visual symmetry grading. Although measured values have been available to graders as a guide for several years, beginning in 2012 GIA will use measured values and apply these boundary limits strictly when grading symmetry for round brilliant cut diamonds. Facet-related symmetry features, and the manner in which multiple features combine, may also affect the symmetry grade, and these aspects will continue to be evaluated visually, as they are presently beyond reproducible instrument measurement. Compared to visual assessment, instrumental measurements provide a more consistent way of establishing a symmetry grade, especially when a diamond has very subtle symmetry deviations. Figure 1 shows a diamond with several symmetry flaws: a wavy and uneven girdle (resulting in an uneven crown height), a table not parallel to the girdle, and uneven bezel facets. In the past, the only means of Since 2006, GIA has used certain proportion measurements obtained with non-contact optical scanners to grade the cut of round brilliant diamonds. Improvements in the operation and accuracy of these instruments now enable us to also measure some symmetry parameters during the grading process. Although both Excellent and Very Good symmetry grades meet GIA’s criteria for an Excellent cut grade (Moses et al., 2004), there is a premium for what the trade calls a “triple Excellent”: an Excellent grade for cut, polish, and symmetry. Therefore, many diamond manufacturers would like to be able to predict GIA symmetry grades from measurement data, so they can apply these consider-