Agate
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Colour:
colorless, gray, red, ...
Lustre:
Waxy, Dull
Hardness:
6½ - 7
Name:
The name is derived from its occurrence at the Achates River in southwestern Sicily.
A variety of Chalcedony
A distinctly banded fibrous chalcedony. Originally reported from Dirillo river (Achates river), Acate, Ragusa Province, Sicily, Italy.
The banding in agate is based on periodic changes in the translucency of the agate substance. Layers appear darker when they are more translucent (this may appear reversed in transmitted light). This effect may be accompanied and amplified by changes in the color of neighboring layers, due to other co-precipitated minerals. In old agates that have been subject to weathering and chemical alteration the differences in translucency may disappear, such specimens may turn almost opaque.
Agate is made of fibrous length-fast chalcedony, sometimes with layers of quartzine (length-slow chalcedony) fibers (Michel-Lévy and Munier-Chalmas 1892; Correns and Nagelschmidt, 1933; Bernauer 1927; Braitsch 1957; Frondel, 1978; Flörke et al. 1991; Graetsch 1994). In thin slices of agate the fibers are sometimes visible in transmitted light and may cause interesting optical effects (see Iris Agate). Because the individual crystallites in the fibers are not tightly interlocked, agate is slightly porous (e.g., Monroe, 1964), and can be dyed easily.
Two characteristic types of banding can be distinguished in agates (e.g. Graetsch 1994):
1. Wall-lining Banding. The individual bands run perpendicular to the orientation and growth direction of the chalcedony fibers. Since the chalcedony fibers grow from the walls to the interior of a cavity, a concentric, onion-like pattern develops. The changes in translucency reflect periodic changes of crystallite sizes and repetitive nucleation of new fibers at the growth front (Taijing and Sunagawa 1994; Cady et al 1998), as well as chemical composition (Frondel 1978; Heaney and Davis 1995). In addition to the visible bands, there are compositional bands of varying trace element and hydroxyl concentrations on the micrometer scale (Frondel 1978, 1985).
Note that this type of banding is not restricted to walls of geodes: similar looking patterns of banding will develop around other structures that grew into the cavity, like crystals or moss-like inclusions.
2. Horizontal Banding (also called Uruguay-type banding). This type of banding is less common, and usually accompanied by wall-lining banding. The banding consists of fine, irregularly spaced layers of small chalcedony spherulites and sometimes quartz crystals that precipitated in the cavity. Horizontal bands can serve as spirit levels to determine the original orientation of the specimen in the host rock. When the difference in translucency or color between the layers is pronounced, agates with horizontal banding can be used for cutting cameos and engravings.
Note: Agate is not simply "banded chalcedony." There are other types of chalcedony that are banded that do not match the description above, banded flint, for example.
Because the colors and patterns found in agates are so varied and so characteristic for the respective localities, there is a confusingly large number of ever-changing varietal and trade names. Roger Pabian's "Agate Lexicon" at UNL is a good source:
Agates Lexicon http://snr.unl.edu/data/geologysoils/agates/agateslexicon.aspx
There are a number of varieties of chalcedony that are called "agate" that do not match the definition given above. Good examples are "feather agates" and "fire agates". These are listed as varieties of chalcedony, not as varieties of agate.
Visit gemdat.org for gemological information about Agate.
A distinctly banded fibrous chalcedony. Originally reported from Dirillo river (Achates river), Acate, Ragusa Province, Sicily, Italy.
The banding in agate is based on periodic changes in the translucency of the agate substance. Layers appear darker when they are more translucent (this may appear reversed in transmitted light). This effect may be accompanied and amplified by changes in the color of neighboring layers, due to other co-precipitated minerals. In old agates that have been subject to weathering and chemical alteration the differences in translucency may disappear, such specimens may turn almost opaque.
Chalcedony fibers and concentric wall-lining banding visible in a thin slice of agate
Agate is made of fibrous length-fast chalcedony, sometimes with layers of quartzine (length-slow chalcedony) fibers (Michel-Lévy and Munier-Chalmas 1892; Correns and Nagelschmidt, 1933; Bernauer 1927; Braitsch 1957; Frondel, 1978; Flörke et al. 1991; Graetsch 1994). In thin slices of agate the fibers are sometimes visible in transmitted light and may cause interesting optical effects (see Iris Agate). Because the individual crystallites in the fibers are not tightly interlocked, agate is slightly porous (e.g., Monroe, 1964), and can be dyed easily.
Two characteristic types of banding can be distinguished in agates (e.g. Graetsch 1994):
1. Wall-lining Banding. The individual bands run perpendicular to the orientation and growth direction of the chalcedony fibers. Since the chalcedony fibers grow from the walls to the interior of a cavity, a concentric, onion-like pattern develops. The changes in translucency reflect periodic changes of crystallite sizes and repetitive nucleation of new fibers at the growth front (Taijing and Sunagawa 1994; Cady et al 1998), as well as chemical composition (Frondel 1978; Heaney and Davis 1995). In addition to the visible bands, there are compositional bands of varying trace element and hydroxyl concentrations on the micrometer scale (Frondel 1978, 1985).
Note that this type of banding is not restricted to walls of geodes: similar looking patterns of banding will develop around other structures that grew into the cavity, like crystals or moss-like inclusions.
2. Horizontal Banding (also called Uruguay-type banding). This type of banding is less common, and usually accompanied by wall-lining banding. The banding consists of fine, irregularly spaced layers of small chalcedony spherulites and sometimes quartz crystals that precipitated in the cavity. Horizontal bands can serve as spirit levels to determine the original orientation of the specimen in the host rock. When the difference in translucency or color between the layers is pronounced, agates with horizontal banding can be used for cutting cameos and engravings.
Note: Agate is not simply "banded chalcedony." There are other types of chalcedony that are banded that do not match the description above, banded flint, for example.
Because the colors and patterns found in agates are so varied and so characteristic for the respective localities, there is a confusingly large number of ever-changing varietal and trade names. Roger Pabian's "Agate Lexicon" at UNL is a good source:
Agates Lexicon http://snr.unl.edu/data/geologysoils/agates/agateslexicon.aspx
There are a number of varieties of chalcedony that are called "agate" that do not match the definition given above. Good examples are "feather agates" and "fire agates". These are listed as varieties of chalcedony, not as varieties of agate.
Visit gemdat.org for gemological information about Agate.
Classification of Agate
Physical Properties of Agate
Waxy, Dull
Diaphaneity (Transparency):
Translucent
Comment:
vitreous when polished
Colour:
colorless, gray, red, white, any color due to embedded minerals, multicolored specimen not uncommon
Streak:
white
Hardness (Mohs):
6½ - 7
Tenacity:
Brittle
Cleavage:
None Observed
Fracture:
Conchoidal, Sub-Conchoidal
Density:
2.6 g/cm3 (Measured)
Comment:
varies with amount and type of impurities
Occurrences of Agate
Relationship of Agate to other Species
Common Associates:
Other Names for Agate
Name in Other Languages:
Dutch:Agaat
Finnish:Akaatti
German:Achat
Italian:Agata
Agata-diasporo
Agata-diasporo
Japanese:玉髄
Latin:Achates vix pellucida, nebulosa, colore griseo mixto
Agathe vixpellucida nebulosa colgriseo ore mixta
Agathe vixpellucida nebulosa colgriseo ore mixta
Simplified Chinese:玛瑙
Other Information
Health Risks:
No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.
References for Agate
Reference List:
Michel-Lévy, A., Munier-Chalmas, C.P.E. (1892) Mémoire sur diverses formes affectées par le réseau élémentaire du quartz. Bulletin de la Société Française de Minéralogie: 15: 159-190.
Bernauer, F. (1927) Über Zickzackbänderung (Runzelbänderung) und verwandte Polarisationserscheinungen an Kristallen und Kristallaggregaten. Neues Jahrbuch für Mineralogie, Geologie und Paläontologie, Beilageband: 55: 92-143.
Correns, C.W., Nagelschmidt, G. (1933) Über Faserbau und optische Eigenschaften von Chalzedon. Zeitschrift für Kristallographie, 85, 199-213.
Jones, F.T. (1952) Iris Agate. American Mineralogist: 37: 578-587.
Braitsch, O. (1957) Über die natürlichen Faser- und Aggregationstypen beim SiO2, ihre Verwachsungsformen, Richtungsstatistik und Doppelbrechung. Heidelberger Beiträge zur Mineralogie und Petrographie: 5: 331-372.
Monroe, E.A. (1964) Electron optical observations of fine-grained silica minerals. American Mineralogist: 49: 339-347.
Frondel, C. (1978) Characters of quartz fibers. American Mineralogist: 63: 17-27.
Frondel, C. (1982) Structural hydroxyl in chalcedony (type B quartz). American Mineralogist: 67: 1248-1257.
Fallick, A.E., Jocelyn, J., Donelly, T., Guy, M., Behan, C. (1985) Origin of agates in volcanic rocks from Scotland. Nature: 313: 672-674.
C. Frondel (1985) Systematic compositional zoning in the quartz fibers of agate. American Mineralogist: 70: 975-979.
Flörke, O.W., Graetsch, H., Martin, B., Röller, K., Wirth, R. (1991) Nomenclature of micro- and non-crystalline silica minerals based on structure and microstructure. Neues Jahrbuch für Mineralogie - Abhandlungen: 163: 19-42.
Heaney, P.J., Post, J.E. (1992) The widespread distribution of a novel silica polymorph in microcrystalline quartz varieties. Science: 255: 441-443.
Graetsch, H. (1994) Structural characteristics of opaline and microcrystalline silica minerals. Reviews in Mineralogy, Vol.29, Silica - Physical behavior, geochemistry and materials applications.
Heaney, P.J., Veblen, D.R., Post, J.E. (1994) Structural disparities between chalcedony and macrocrystalline quartz. American Mineralogist: 79: 452-460.
Taijing, L. Sunagawa, I. (1994) Texture formation of agate in geode. Mineralogical Journal: 17: 53-76.
Tanaka, T., Kamioka, H. (1994) Trace element abundance in agate. Geochemical Journal: 28: 359-362.
Taijing, L., Zhang, X. (1995) Nanometer scale textures in agate and Beltane opal. Mineralogical Magazine: 59: 103-109.
Heaney, P.J., Davis, A.M. (1995) Observation and origin of self-organized textures in agates. Science: 269: 1562-1565.
Merino, E., Wang, Y., Deloule, E. (1995) Genesis of agates in flood basalts: twisting of chalcedony fibers and trace-element geochemistry. American Journal of Science: 295: 1156-1176.
Wang, Y., Merino, E. (1995) Origin of fibrosity and banding in agates from flood basalts. American Journal of Science: 295: 49-77.
Cady, S.L., Wenk, H.R., Sintubin, M. (1998) Microfibrous quartz varieties: characterization by quantitative X-ray texture analysis and transmission electron microscopy. Contributions to Mineralogy and Petrology: 130: 320-335.
Götze, J., Plötze, M., Fuchs, H., and Habermann, D. (1999) Defect structure and luminescence behavior of agate - results of electron paramagnetic resonance (EPR) and cathodoluminescence (CL) studies. Mineralogical Magazine: 63: 149-163.
Weise, C., publisher (2000) Achat - extraLapis Nr.19. Christian Weise Verlag, München.
Carlson, M.R. (2002) The Beauty of Banded Agates. Edina: Fortification Press.
Moxon, T., Rios, S. (2002) Agate: a study of ageing. European Journal of Mineralogy: 14: 1109-1118.
Moxon, T. (2004) Moganite and water content as a function of age in agate: an XRD and thermogravimetric study. European Journal of Mineralogy, 16, 269-278.
Zenz, J. (2005) Achate. Bode Verlag, 656 pp. (in German).
Moxon, T., Reed, S.J.B. (2006) Agate and chalcedony from igneous and sedimentary hosts aged from 13 to 3480 Ma: a cathodoluminescence study. Mineralogical Magazine: 70: 485-498.
Zenz, J. (2009) Achate II. Bode Verlag, 656 pp. (in German).
Moxon, T., Carpenter, M.A. (2009) Crystallite growth kinetics in nanocrystalline quartz (agate and chalcedony). Mineralogical Magazine: 73: 551-568.
Moxon, T. (2009): Studies on Agate: Microscopy, Spectroscopy, Growth, High Temperature and Possible Origin. Terra Publications, Doncaster, UK, 96 pp.
Weise, C., publisher (2010) Achate - geboren aus Vulkanen. extraLapis Nr.39. Christian Weise Verlag, München, 98pp.
Götze, J., Gaft, M., Möckel, R. (2015) Uranium and uranyl luminescence in agate/chalcedony. Mineralogical Magazine: 79: 985-995.
http://snr.unl.edu/data/geologysoils/agates/agateslexicon.aspx [Agates Lexicon]
http://www.achat-almanach.de [in English and German]
Bernauer, F. (1927) Über Zickzackbänderung (Runzelbänderung) und verwandte Polarisationserscheinungen an Kristallen und Kristallaggregaten. Neues Jahrbuch für Mineralogie, Geologie und Paläontologie, Beilageband: 55: 92-143.
Correns, C.W., Nagelschmidt, G. (1933) Über Faserbau und optische Eigenschaften von Chalzedon. Zeitschrift für Kristallographie, 85, 199-213.
Jones, F.T. (1952) Iris Agate. American Mineralogist: 37: 578-587.
Braitsch, O. (1957) Über die natürlichen Faser- und Aggregationstypen beim SiO2, ihre Verwachsungsformen, Richtungsstatistik und Doppelbrechung. Heidelberger Beiträge zur Mineralogie und Petrographie: 5: 331-372.
Monroe, E.A. (1964) Electron optical observations of fine-grained silica minerals. American Mineralogist: 49: 339-347.
Frondel, C. (1978) Characters of quartz fibers. American Mineralogist: 63: 17-27.
Frondel, C. (1982) Structural hydroxyl in chalcedony (type B quartz). American Mineralogist: 67: 1248-1257.
Fallick, A.E., Jocelyn, J., Donelly, T., Guy, M., Behan, C. (1985) Origin of agates in volcanic rocks from Scotland. Nature: 313: 672-674.
C. Frondel (1985) Systematic compositional zoning in the quartz fibers of agate. American Mineralogist: 70: 975-979.
Flörke, O.W., Graetsch, H., Martin, B., Röller, K., Wirth, R. (1991) Nomenclature of micro- and non-crystalline silica minerals based on structure and microstructure. Neues Jahrbuch für Mineralogie - Abhandlungen: 163: 19-42.
Heaney, P.J., Post, J.E. (1992) The widespread distribution of a novel silica polymorph in microcrystalline quartz varieties. Science: 255: 441-443.
Graetsch, H. (1994) Structural characteristics of opaline and microcrystalline silica minerals. Reviews in Mineralogy, Vol.29, Silica - Physical behavior, geochemistry and materials applications.
Heaney, P.J., Veblen, D.R., Post, J.E. (1994) Structural disparities between chalcedony and macrocrystalline quartz. American Mineralogist: 79: 452-460.
Taijing, L. Sunagawa, I. (1994) Texture formation of agate in geode. Mineralogical Journal: 17: 53-76.
Tanaka, T., Kamioka, H. (1994) Trace element abundance in agate. Geochemical Journal: 28: 359-362.
Taijing, L., Zhang, X. (1995) Nanometer scale textures in agate and Beltane opal. Mineralogical Magazine: 59: 103-109.
Heaney, P.J., Davis, A.M. (1995) Observation and origin of self-organized textures in agates. Science: 269: 1562-1565.
Merino, E., Wang, Y., Deloule, E. (1995) Genesis of agates in flood basalts: twisting of chalcedony fibers and trace-element geochemistry. American Journal of Science: 295: 1156-1176.
Wang, Y., Merino, E. (1995) Origin of fibrosity and banding in agates from flood basalts. American Journal of Science: 295: 49-77.
Cady, S.L., Wenk, H.R., Sintubin, M. (1998) Microfibrous quartz varieties: characterization by quantitative X-ray texture analysis and transmission electron microscopy. Contributions to Mineralogy and Petrology: 130: 320-335.
Götze, J., Plötze, M., Fuchs, H., and Habermann, D. (1999) Defect structure and luminescence behavior of agate - results of electron paramagnetic resonance (EPR) and cathodoluminescence (CL) studies. Mineralogical Magazine: 63: 149-163.
Weise, C., publisher (2000) Achat - extraLapis Nr.19. Christian Weise Verlag, München.
Carlson, M.R. (2002) The Beauty of Banded Agates. Edina: Fortification Press.
Moxon, T., Rios, S. (2002) Agate: a study of ageing. European Journal of Mineralogy: 14: 1109-1118.
Moxon, T. (2004) Moganite and water content as a function of age in agate: an XRD and thermogravimetric study. European Journal of Mineralogy, 16, 269-278.
Zenz, J. (2005) Achate. Bode Verlag, 656 pp. (in German).
Moxon, T., Reed, S.J.B. (2006) Agate and chalcedony from igneous and sedimentary hosts aged from 13 to 3480 Ma: a cathodoluminescence study. Mineralogical Magazine: 70: 485-498.
Zenz, J. (2009) Achate II. Bode Verlag, 656 pp. (in German).
Moxon, T., Carpenter, M.A. (2009) Crystallite growth kinetics in nanocrystalline quartz (agate and chalcedony). Mineralogical Magazine: 73: 551-568.
Moxon, T. (2009): Studies on Agate: Microscopy, Spectroscopy, Growth, High Temperature and Possible Origin. Terra Publications, Doncaster, UK, 96 pp.
Weise, C., publisher (2010) Achate - geboren aus Vulkanen. extraLapis Nr.39. Christian Weise Verlag, München, 98pp.
Götze, J., Gaft, M., Möckel, R. (2015) Uranium and uranyl luminescence in agate/chalcedony. Mineralogical Magazine: 79: 985-995.
http://snr.unl.edu/data/geologysoils/agates/agateslexicon.aspx [Agates Lexicon]
http://www.achat-almanach.de [in English and German]
Internet Links for Agate
mindat.org URL:
https://www.mindat.org/min-51.html
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The following Agate specimens are currently listed for sale on minfind.com.
Ojo Laguna, Estacion Ojo Laguna, Mun. de Ahumada, Chihuahua, Mexico