H6449-Prelims.qxd 7/13/05 10:49 AM Page i Gemmology H6449-Prelims.qxd 7/13/05 10:49 AM Page ii Related titles The Diamond Formula A. Barnard Gem Testing, Tenth edition B.W. Anderson, Revised by E.A. Jobbins Gems, Sixth edition M. O’Donoghue Gemstone Enhancement, Second edition K. Nassau Synthetic, Imitation and Treated Gemstones M. O’Donoghue Front cover: Amethyst and diamond necklace on amethyst crystal (Courtesy of P.J. Watson) H6449-Prelims.qxd 7/13/05 10:49 AM Page iii Gemmology Third edition P.G. Read AMSTERDAM • BOSTON • HEIDELBERG • LONDON • NEW YORK • OXFORD PARIS • SAN DIEGO • SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO H6449-Prelims.qxd 7/13/05 10:49 AM Page iv Elsevier Butterworth-Heinemann Linacre House, Jordan Hill, Oxford OX2 8DP 30 Corporate Drive, Burlington, MA 01803 First published 1991 Paperback edition 1995 Reprinted 1997, 1998 Second edition 1999 Reprinted 2000, 2003, 2004, 2005 Third edition 2005 Copyright © 1999, 2005 Elsevier Ltd. All rights reserved. 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British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0 7506 6449 5 For information on all Elsevier Butterworth-Heinemann publications visit our website at www.bh.com Typeset by Charon Tec Pvt. Ltd, Chennai, India www.charontec.com Printed and bound in Great Britain H6449-Prelims.qxd 7/13/05 10:49 AM Page v Contents Preface x Acknowledgements xi 1 Introduction 1 The evolution of the science of gemmology 1 Highlights of the last 170 years 1 The essential qualities of a gem material 9 Organic and inorganic gems 10 2 The geological origin, occurrence and locality of gemstones 11 The Earth’s structure 11 Minerals and rocks 12 The formation of rocks in the Earth’s crust 12 Mineral groups, species and varieties 13 The origin of gems 13 Gem occurrences 16 The major gem localities 16 Mining techniques 18 3 The chemical composition of gemstones 22 Atoms, elements, molecules and compounds 22 Valency 24 Bonding 25 The relationship between chemical composition and durability 27 4 Crystallography 29 Crystalline and non-crystalline materials 29 The atomic structure of a crystal 31 Classification of crystals by symmetry 33 The seven crystal systems and their elements of symmetry 34 Crystal forms 38 Crystal habits and their use in identification 39 Twinned crystals 40 Polycrystalline and microcrystalline minerals 42 Metamict minerals 42 v H6449-Prelims.qxd 7/13/05 10:49 AM Page vi vi Contents 5 Cleavage, parting and fracture 43 The mechanism of cleavage 43 Lapidary problems caused by cleavage planes 44 The use of cleavage in the fashioning of diamond 46 Parting (false or pseudo-cleavage) 46 Fracture and its use as an aid to identification 46 6 Hardness 48 The durability of a gemstone 48 Mohs’ scale of comparative hardness 49 Hardness tests (using hardness pencils and plates) 49 Directional hardness 51 Engineering hardness tests 52 The influence of hardness on mining techniques 53 7 Specific gravity, density and relative density 54 Definitions 54 Archimedes’ principle and the measurement of SG 54 Measurement of SG by displacement 55 Hydrostatic methods of SG measurement 57 Approximation of SG using heavy liquids 61 Precise measurement of SG using heavy liquids 65 Sodium polytungstate 65 Safety precautions 66 8 Colour, lustre and sheen 67 The electromagnetic spectrum 67 The photon 68 Colour and selective absorption 69 Allochromatic and idiochromatic gems 70 The transition elements 70 Colour-change gemstones 71 Interference colours 72 Dispersion 72 Colour centres 73 Lustre 74 Sheen 74 Colour, transparency and identification 77 9 Reflection and refraction 79 Snell’s laws of reflection and refraction 79 Refractive index 80 Double refraction 81 The refractometer 82 Contact fluid 84 Sources of illumination 84 Using the refractometer 85 The Dialdex refractometer 88 Optic axes, optic sign and optical character 88 H6449-Prelims.qxd 7/13/05 10:49 AM Page vii Contents vii Distant vision method 91 Special refractometer versions 92 Fresnel’s reflectivity equation 93 The reflectance meter 94 Other methods of RI measurement 97 10 Polarization and pleochroism 105 The polarization of light 105 Polarizing filters 105 The polariscope 107 Using the polariscope 109 Pleochroism (differential selective absorption) 111 The dichroscope 112 11 Spectroscopy 115 Absorption and emission spectra 115 The prism spectroscope 116 The diffraction grating spectroscope 119 Using the spectroscope 120 Fine line spectra 123 Fraunhofer lines 124 Other spectroscope versions 125 Raman spectroscopy 125 Spectrophotometers 126 12 Luminescent, electrical and thermal properties of gemstones 127 Luminescence (fluorescence and phosphorescence) 127 Photoluminescence and Stokes’ law 128 Crossed filters 128 LW and SW UV radiation 130 X-ray radiation 132 Other forms of luminescence 133 SW UV transmission test 134 Near-UV transparency tester 135 Electrical properties 135 Thermal conductivity and thermal inertia 137 Thermal conductance testers 138 13 The hand lens, microscope and Chelsea filter 141 Types of hand lens 141 Types of microscope 143 Light-field, dark-field and incident illumination 145 Pin-point illumination and shadowing techniques 147 Immersion techniques 147 Additional uses for the microscope 148 Care in the use of the microscope 150 The Chelsea filter 151 Handling gemstones 152 H6449-Prelims.qxd 7/13/05 10:49 AM Page viii viii Contents 14 Gemstone enhancement 155 Foils, colour backing and dyeing 155 Impregnation 156 Heat treatments 157 Surface diffusion of corundum 160 Lattice diffusion of corundum 161 HPHT enhancement of diamond 162 Irradiation methods 162 Glass filling 165 Laser drilling of diamond 166 Disclosure of enhancements 167 15 Synthetic gemstones and gemstone simulants 168 A brief history of early gemstone synthesis 168 Definitions 169 The Verneuil flame-fusion process 169 The Czochralski method 172 Flux-melt growth 173 Zone melting 176 The hydrothermal process 176 The skull-crucible process 179 Diamond synthesis 181 Thin diamond films 184 Chemical vapour deposition (CVD) 184 Synthetic jadeite 185 ‘Synthetic’ lapis lazuli, turquoise and opal 185 Composite gemstones 186 16 Identifying inorganic gemstones and their synthetic counterparts 189 Growth lines and colour zoning 189 Twinning 189 Types of inclusion 190 Identification features of natural and synthetic gemstones 190 Colour zone diffusion and induced feathers/fingerprints 204 Use of the polariscope (identifying synthetic quartz and Verneuil corundum) 204 Laboratory equipment and methods 205 17 Identifying inorganic gemstone simulants 212 Alexandrite 213 Aquamarine 213 Diamond 214 Additional tests for diamond 215 Emerald 218 Jade 218 Lapis lazuli 219 Turquoise 219 Opal 220 Quartz 221 H6449-Prelims.qxd 7/13/05 10:49 AM Page ix Contents ix Ruby 221 Sapphire 222 18 Identifying organic gem materials and their simulants 223 Natural pearls 223 Cultured pearls 225 Distinguishing natural from cultured pearls 228 Imitation pearls and their identification 231 Shell 234 Amber 235 Ivory 237 Ondontolite 239 Tortoiseshell 239 Jet 240 Coral 241 19 The fashioning of gemstones 243 Critical angle 243 Critical angle and the jewellery owner 246 Cutting styles 246 Gemstone polishing 252 Lapidary techniques 252 Gemstone carving 254 Diamond polishing methods 256 Diamond grading 259 20 Practical gemstone identification 263 Observation 264 Selecting the appropriate tests 265 Appendices 267 A Bibliography 267 B The organic gems 268 C The inorganic gems 271 D Man-made diamond simulants 292 E Examination notes 294 F Gemstone constants 297 G Units of measurement 303 H Table of elements 304 I Table of principal Fraunhofer lines 306 J Gemstone weighing 306 Index 309 H6449-Prelims.qxd 7/13/05 10:49 AM Page x Preface My Beginner’s Guide to Gemmology was first published in 1980, and although this was only intended as an introduction to the subject, it was apparently used with some success as a textbook by many students. For several years I had acted as tutor to stu- dents taking the Gemmological Association’s correspondence courses, and this experi- ence prompted me in 1991 to produce a more expansive and up-to-date volume in the original hardback edition of Gemmology. Both the first and second editions of the book were written with the aim of provid- ing a readable account of the relatively modern science of gemstones as well as forming a text to assist students preparing for the Gemmological Association’s Preliminary and Diploma examinations. A similar format has been retained in this third edition, but more emphasis has been placed on the use of the book as a work of reference. To this end the index has been enlarged, and cross-links between chapters have been increased. The replacement of the original Preliminary course examination by the new Foundation course examin- ation (and its associated practical endorsement requirement) is covered in Appendix E – Examination notes. In line with the increased practical content of the Foundation course, more emphasis has been placed on the identification of natural gemstones, their simulants and their synthetic counterparts.