Agates
Inside Out
Designed, Written, and Compiled by Karen A. Brzys
Photography by Thomas P. Shearer
Research Assistance by Candace Prill
Cover Photograph: An intricate Lake Superior agate.
Figure 1 (above): A condor agate from Argentina.
Table of Contents
Dedication ...... vi
Acknowledgements...... vii
Introduction...... viii
CHAPTER 1: What Is An Agate? ...... 1
A BASIC DESCRIPTION...... 1
AGATES AND HUMAN HISTORY...... 2
AGATES AND THEIR “FIRST COUSINS”...... 4
Macrocrystalline Quartz ...... 4
Fibrous Microcrystalline Quartz ...... 8
Granular Microcrystalline Quartz...... 9
Silicified Fossils...... 10
QUARTZ PROPERTIES...... 14
CHAPTER 2: How Did Agates Form? ...... 16
THE EARTH — IN THE BEGINNING...... 16
AGATE FORMATION ...... 21
INFLOW AGATE GENESIS THEORY...... 24
Possible Sources of Silica...... 28
Silica Transport ...... 31
Infiltration Channels ...... 34
Concentric Banding...... 41
Horizontal Banding...... 55
SILICA GEL AGATE GENESIS THEORY...... 57
AGATE GENESIS CONCLUSIONS ...... 60
Silica Gel Theory ...... 60
Inflow Theory ...... 62
TYPES OF HOST ROCK...... 65
Igneous Rock Agates ...... 67
Amygdaloidal Agates ...... 67
Rhyolitic Agates ...... 70
Sedimentary Rock Agates ...... 73
TIMING OF AGATE FORMATION...... 76
CHAPTER 3: How Do You Find Agates? ...... 82
A POCKET FILLED IN...... 88
BANDING AND OTHER PATTERNS...... 90
TRANSLUCENCY...... 93
COLOR...... 95
SHAPE, SIZE, AND WEIGHT...... 106
LUSTER...... 109
HUSK...... 112
CONNECTIVE CHANNELS...... 118
PSEUDOBANDS...... 120
AGATE-WANT-TO-BE...... 122
Chert , Flint, and Jasper...... 122
Metamorphic Rocks ...... 126
Secondary Fills...... 127
Silicified Fossils...... 128
Other Agates -Want-To-Be...... 132
CARING FOR AND CLEANING AGATES...... 135
ASSESSING THE VALUE OF AGATES ...... 138
CHAPTER 4: What Are the Different Types of Agate? ...... 139
BOTRYOIDAL (GRAPE) AGATES...... 139
BRECCIATED (MOSAIC) AGATES...... 140
CANDY STRIPED AGATES ...... 141
CARNELIAN AGATES...... 142
CLOUD AGATES...... 144
CRAZY LACE AGATES...... 144
DENDRITIC AGATES...... 146
ENHYDRO (WATER-FILLED) AGATES ...... 150
EYE AGATES ...... 151
FIRE AGATES...... 156
FLAME AGATES...... 158
FLOATER BAND AGATES ...... 159
FORTIFICATION AGATES ...... 159
GEODE AGATES...... 161
IRIS (RAINBOW) AGATES...... 162
JASPER AGATES...... 165
MOSS AGATES...... 165
PAINTSTONE AGATES...... 168
PEELER AGATES...... 168
PLUME AGATES...... 170
POLYHEDROID AGATES...... 172
PSEUDOMORPH AGATES...... 173
RUIN AGATES...... 174
SAGENITE AGATES...... 175
SARDONYX...... 177
SEAM AGATES...... 178
SHADOW AGATES...... 180
STALK AGGREGATE AGATES...... 182
Thunder Egg Agates ...... 182
Tube Agates ...... 184
Water -Level Agates ...... 186
CHAPTER 5: Where Can You Find Agates?...... 187
A SELECTION OF UNITED STATES AGATES...... 187
California...... 190
Florida...... 191
Idaho...... 192
Iowa ...... 193
Kentucky...... 194
Michigan...... 196
Minnesota ...... 198
Missouri...... 200
Montana ...... 201
New Mexico...... 203
Oregon...... 204
South Dakota ...... 207
Texas...... 210
Wyoming...... 211
A SELECTION OF INTERNATIONAL AGATES ...... 212
Argentina...... 212
Australia...... 215
Botswana , Africa...... 216
Brazil...... 218
Canada...... 220
Caribbean ...... 223
China...... 225
Mexico...... 227
References...... 231
Index ...... 239
MUSEUM INFORMATION ...... 243
HOW TO VOTE FOR YOUR FAVORITE PHOTOgraph...... 244
INFORMATION ABOUT AGATE DVDS & OTHER PRODUCTS244
Rock hunting log ...... 245
CHAPTER 1 What Is An Agate?
A BASIC DESCRIPTION
One of the most intriguing forms of quartz produced by nature is the semi-precious gemstone, agate. It is a variegated form of chalcedony (pronounced kal-sed’-nee), which is a type of microcrystalline quartz made of silicon dioxide (SiO2).2 Microcrystalline means that the crystals are so small that they cannot be seen with the naked eye or with a normal optical microscope.3 In fact, the only way to see actual agate crystals is to use powerful technology such as transmission electron microscopy TEM or scanning electron microscopy SEM.4
Agates naturally develop when empty pockets or veins inside host rock fill in molecule-by-molecule and layer-by-layer as quartz microcrystals self-organize to form concentric bands or other patterns. Agates can be found in igneous, sedimentary, and metamorphic rocks and have been discovered on every continent.5 They can be collected from deserts, mountains, rivers, shorelines, gravel pits, prairies, and fields.
Agates are technically classified as sedimentary because they are deposited in empty pockets with silica from mineral-rich fluids or gels.6The colors and arrangement of microcrystals are influenced by changes in pressure, temperature, and mineral content that occur during the formation process.7 Unlike other gemstones, each agate is unique. Even slabs cut from the same specimen will vary.