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Summer 1999 Gems & Gemology

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Summer 1999 Gems & Gemology SUMMER 1999 VOLUME 35 NO. 2 TABLE OF CONTENTS EDITORIAL 79 Nothing Is as Simple as It Seems Alice S. Keller 80 LETTERS FEATURE ARTICLES 82 On the Identification of Various Emerald Filling Substances pg. 83 Mary L. Johnson, Shane Elen, and Sam Muhlmeister 108 Sapphire and Garnet from Kalalani, Tanga Province, Tanzania Antonin V. Seifert and Jaroslav Hyrsl 122 Russian Synthetic Ametrine Vladimir S. Balitsky, Taijin Lu, George R. Rossman, Irina B. Makhina, Anatolii A. Mar’in, James E. Shigley, Shane Elen, and Boris A. Dorogovin REGULAR FEATURES 135 1999 Challenge Winners pg. 109 136 Gem Trade Lab Notes 142 Gem News 156 Thank You, Donors 157 Book Reviews 160 Gemological Abstracts ABOUT THE COVER: Emeralds typically contain numerous fractures when recovered, and yet consumers have high expectations for their clarity as well as color. As a result, the overwhelming majority of emeralds on the market today have undergone some form of clarity enhancement. The proliferation of filler substances in recent years has raised widespread concern in the trade about their identity and durability. The lead article in this issue reviews the history of emerald clarity enhancement and the current situation in the trade. It then focuses on possible methods of distinguishing among different clari- pg. 130 ty-enhancement substances. The emerald cabochon in this Sabi-finish gold necklace weighs 77.68 ct. Courtesy of pg. 145 Henry Dunay Designs Inc., New York. Photo © Harold & Erica Van Pelt––Photographers, Los Angeles, California. Color separations for Gems & Gemology are by Pacific Color, Carlsbad, California. Printing is by Fry Communications Inc., Mechanicsburg, Pennsylvania © 1999 Gemological Institute of America All rights reserved. ISSN 0016-626X Nothing Is as Simple as It Seems omeone asked me recently if gemologists really used tion for most others. Once the determination of, for exam- the infrared, EDXRF, and Raman spectra that we now ple, R.I. and S.G. has narrowed the field, microscopic fea- S publish in most Gems & Gemology articles. Twenty tures often will reveal strong indications of natural or syn- years ago, a good gemologist could identify almost any gem thetic origin, or whether the stone has been treated. Yet even material, and most enhancement processes, with straightfor- the field of microscopy has changed. In many cases where ward gemological testing— refractive index, specific gravity, standard inclusions or color zoning cannot conclusively iden- optic character, and the like. Twenty years ago, we did quite tify ‘flawless’ synthetic rubies and sapphires, growth structure well (some tell me) without all of this advanced instrumenta- analysis can make the distinction, but this requires a new tion. A new material like cubic zirconia could be addressed level of technical knowledge and skill with a microscope. quickly and efficiently. The major issues were gem localities, The article on the identification of emerald fillers in this cause of color, and interesting new inclusions. Twenty years issue is a good case in point. As the authors note, there are tens ago, we did not have commercial quantities of flux and of thousands of potential emerald fillers. They studied 39 hydrothermal synthetic sap- known and possible filler sub- phires, gem-quality synthetic dia- stances—and found that to make monds, or bleached and impreg- even basic distinctions between ‘pre- nated ‘B’ jade, and few questions sumed natural’ oils and artificial were asked about the filler used resins required a combination of in ‘oiled’ emeralds. techniques, such as microscopy and Think about it. Ask yourself infrared or Raman spectroscopy. how many gem materials pre- Even the advanced techniques cannot sented to you in a simple white always resolve the challenges posed by parcel paper you could identify emeralds that have undergone more conclusively — the material, than one generation of treatment (a whether natural or synthetic, the common practice) or have been filled presence and nature of any treat- “Impressive. What does it do?” with mixtures of substances. ment —with the same basic equipment gemologists used Yes, the articles have become more technical and some- almost exclusively twenty years ago. The stone is green. The times can be daunting. And, no, the 21st century gemologist question no longer is ‘simply’ whether it is emerald, peridot, does not have to be a spectroscopist, chemist, or physicist. tourmaline, or the like. But rather, once you know it’s emer- However, he or she should develop some understanding of ald, is it a natural stone or a flux or hydrothermal synthetic? how sophisticated techniques in spectroscopy, chemistry, and Could there be an overgrowth? Is it filled? How extensively? physics can contribute to the gem identification process. As With what? Twenty years ago, the grading of near-colorless new materials and treatments have become more advanced diamonds required primarily determination of color, clarity, and difficult to detect, the means of identification have and finish along with the plotting of inclusions. Now, it become more complicated as well. Like all good profession- must first be determined whether it is diamond or a convinc- als, gemologists must know the limitations of their knowl- ing imitation, and—if diamond—whether it is natural or edge and the outside resources that are available when they synthetic. Then, is it fracture-filled? And eventually, perhaps, reach these limitations. An awareness of the capabilities of does it show evidence of ‘lightening’ by the GE process? advanced testing methods available in modern gem testing Carefully executed standard gemological practices can still laboratories is a must. Because, indeed, nothing is as simple provide identification for many stones and essential informa- as it seems. Alice S. Keller EDITOR, [email protected] Editorial GEMS & GEMOLOGY Summer 1999 79 Heat Exposure May Affect Reflectance faceted synthetic moissanites on three standard reflectance testers (Presidium, DiamondEye, and Jemeter Testing of Synthetic Moissanite Digital 90) and with the C3 Tester Model 590, and veri- fied that they measured as moissanite. (With a thermal In the Winter 1997 Gems & Gemology article by K. conductivity probe, the same samples registered as “dia- Nassau et al., “Synthetic Moissanite: A New Diamond mond.”) The stones were then heated in air for varying Substitute,” it was noted that testing with a reflectivity times using a furnace. Although the times and tempera- meter “can also give diagnostic results” for identifying tures used were more extreme than those encountered in this material, provided great care is taken with the instru- typical jewelry fabrication, they were still well below ment (see p. 273). Recently, however, scientists at C3 Inc., those required to cause damage (e.g., surface pitting visi- the company that manufactures and distributes synthetic ble at 50×–100× magnification) to the stones. The stones moissanite to the gem trade, discovered that exposure of were re-measured on the reflectance testers. Depending gem-quality synthetic moissanite to high temperatures on the time and temperature of heating, the treated sam- for extended periods of time can result in thin oxide films ples of synthetic moissanite measured incorrectly as dia- on the surface that may cause a shift in the measured sur- mond, cubic zirconia, or YAG; longer exposure to the face reflectance. While such coatings are beneficial to heat treatment produced lower values of surface electronics applications of silicon carbide, they can create reflectance as the oxide film grew thicker. The results substantial errors when the material is tested on gem- given by the different reflectance meters were in general stone reflectance meters, potentially causing synthetic agreement. A surface reflectance measurement equivalent moissanite to be mistaken for diamond. to that of diamond was produced by a 40- to 60-nm-thick Most commercial reflectance testers bounce an film of oxide, which is very difficult to detect using com- infrared beam off the surface to be tested and onto a monly available means; no visible changes in the appear- detector that records the intensity of the reflected beam. If ance of the heated stones could be seen. Note that, after the surface is composed of a single (homogeneous) well- treatment, all of the faceted synthetic moissanites still polished material, then the light reflected can be related to the refractive index of that material. If, however, a thin film of another material coats the surface of the gem- stone, the reflectance signal will no longer be related in a Figure 1. The surface reflectance of synthetic simple way to the refractive index of the underlying bulk moissanite is reduced by the thermal growth of an material (see, e.g., E. Hecht and A. Zajac, Optics, Addison oxide film. The measured reflectance value may Wesley Publishing Co., Reading, MA, 1979, p. 311). equal that of diamond or any lower-R.I. gem materi- In the case of synthetic moissanite (silicon carbide, or al, depending on the thickness of the oxide coating SiC), a stable, uniform, thin surface film of silicon dioxide (as shown here for a 45° incident beam angle). (SiO2) is produced when the gem material is heated suffi- ciently in an oxygen-containing atmosphere. The result- ing change in surface reflectance strongly depends on the thickness and composition of the oxide film (figure 1). As the thickness of the film increases, the measured surface reflectance drops through the arbitrarily chosen reflectance values for moissanite, diamond, cubic zirco- nia, and yttrium aluminum garnet (YAG). In fact, when the oxide film reaches a certain thickness, the surface will become antireflective and will register as zero on a reflectance meter. The exact thickness of the oxide film that produces the erroneous diamond reflectance mea- surement will vary according to the precise wavelength of the infrared beam and the incidence angle used by the reflectance tester. We performed some simple experiments to confirm the predicted reflectance behavior.
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