Agate

 

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It  is not easy to explain the term "agate". For most practical purposes it should be sufficient to define agate as a "banded chalcedony".

But the notion of agate also embraces chalcedony variants that do not show any signs of banding, probably because of the long-term use of names like "moss agate" for stones that would simply be more difficult to sell as "moss chalcedony". It is difficult to draw a line between agate and other types of chalcedony.

A chalcedony is usually called an agate if it exhibits any of the following properties:

So an "ideal agate" is a nodule filled with a translucent, multicolored chalcedony with parallel bands. The minimum requirement would be that it is either translucent and exhibts some colored pattern or shows banding.

This is still not the complete story, as structural considerations also play a role in classifying a specimen. A chert can be both multicolored and slightly translucent, but it will not be called an agate, as it lacks certain structural properties that are typically found in agates.

Strictly spoken, agate is not a mineral [1]. Agate does not have a homogeneuos structure, like a crystal, and it usually isn't even made of a single type of mineral. It resembles a rock made up of different components in varying proportions, but I prefer to call it a textural variety of quartz, like all the other cryptocrystalline quartz varieties.

 

Specific Properties

Agate can be of any color, the most frequent colors are (in descending order) gray, white, brown, salmon, red, orange, black, and yellow. Shades of violet or a grayish-blue can occur, deep green and blue tones are very unusual, turquoise-colored specimen come from Needles, California. Specimen from Brazil often lack a vivid coloration and thin slices are artificially "enhanced" with various dyes, yielding deep green, blue, pink, and sometimes more unsuspicious brown tones.

Agate is a porous material, that can be easily be dyed. Occasionally agate geodes are found that still have some of the water captured in a central cavity, so called enhydros. You can sometimes hear the water when you shake the specimen. These will slowly loose their water as it escapes through tiny capillaries and evaporates at the surface. There's nothing special about enhydros except for being quite rare, they simply didn't dry out yet, as all the other agates did. Remember that agates form in a watery environment.

 

Occurrence

Agates typically occur in volcanic rocks. They are sometimes found in sedimentary rocks, while occurrences in metamorphic and igneous rocks are exceptional.


Volcanic Rocks

Agate forms during secondary processes in volcanic rocks, long after these have solidified, and at relatively low temperatures. It fills out cavities in the rock, either isolated geodes of various shapes, or irregular cracks. The shape of the agate nodules also depends on the composition and structure of the volcanic rock.

Finally, agates can fill irregular cracks in the already solidified lava that have been formed during cooling and shrinking of the rock.


Sedimentary Rocks

In addition to the classical agates from volcanic rocks, agate can form as a "vein" agate in and around hydrothermal veins, for example at certain ore deposits. Occasionally, agate can be found in nodules in sedimentary rocks.


Silicified Wood

If wood gets buried by volcanic ashes during eruptions, the wooden substance is often completely replaced by silica, either opal or cryptocrystalline quartz. Small voids in the wood structure and cracks are sometimes filled by agates. Similar processes can also occur in silica-rich sedimentary rocks like sandstone.


 

Agate Varieties

The names of agate varieties are chosen more or less arbitrarily according to their visual appearance, usually that of a cut and polished stone - there are no strict rules or definitions. With such a terminology it is no wonder that there is a countless number of agate "varieties", ->Zenz lists 122 different varietes, for example. A few terms are widespread and people agree on their meaning. Some of the names have very little to with the properties of the agate itself, but with the way the agates have been cut: "eye agate" is probably the best example. A "flame agate" in English is not the same as the literal equivalent "Flammenachat" in German. The same is true for "coral agate" which can be a chalcedony pseudomorph after coral (and thus not really an agate), but also a reddish agate with a certain growth pattern.
Most of the agate names have no mineralogical significance.

 

Further Information, Literature, Links

Johann Zenz has written a very nice book, ->Agates, available in German and English language. It gives an overview of worldwide locations and contains about 2000 images of agates.


 

Locations and Specimen

Of the thousands of agate locations dozens can be considered "classical", and of course it is impossible to cover them comprehensively. The "classical agate countries" are Argentina, Brazil, Germany, Mexico, Morocco, and the U.S.A. I can only present a very small selection of agates.


Botswana


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Agates  don't have to show a symmetrical banding, and agates from Botswana are renown for showing an acentric and fine banding. They occur in weathered basalt rock in eastern Botswana, near the Limpopo river, and are thus also known as "Limpopo Agates".


Brazil


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A  egg-shaped massive geode filled with a very dark agate form an unknown location in Brazil.



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An  "Uruguay agate" with its typical parallel banding that is perfect for cutting cameos, with pale amethyst crystals.



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This  is one of the ever popular ocos that are typically outlined by clear quartz crystals. They are sometimes also named cloud agates, but I'm not sure if that is the appropriate term. It is likely from the Soledade region in Rio Grande Do Sul. There is no real banding visible in these "agates", and if you look at the white clouds closely you will note that the white bands outline former quartz crystal tips. So one could argue whether it is actually agate or chalcedony, and that's why it appears both in the agate and in the chalcedony sections.


Germany


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Juchem  quarry, Niederwörresbach, north east of Idar-Oberstein.



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Juchem  quarry, Niederwörresbach



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Juchem  quarry, Niederwörresbach



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St.  Egidien [Q113]



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 Lierbachtal, Oppenau, Black Forest



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 Lierbachtal, Oppenau, Black Forest



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A  small agate that has been picked up from the fields around Rimsberg at Birkenfeld, Idar. In the lower left corners you see greenish inclusions, probably chlorite.


Italy


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Irregularly  shaped gray chalcedony nodules can be found near Masulas in central Sardegna. Although they are almost uniformly gray in color, they show a very fine and faint banding and thus qualify as agates.


Mexico


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A  Coyamo agate from Aldama at Santa Ulalia, Chihuahua. This is obviously a thunderegg, and not a typical example of Mexican agates (if there is any "typical agate" in a country that yields so many different types of agates).

The black minerals in the upper corner are manganese oxides (like pyrolusite, with tetravalent manganese Mn4+) that also stain the outer part of the agate gray, while bivalent manganese compounds with Mn2+ ions give the inner agate layers a faint pink color.


Morocco


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A  vein agate that filled out a crack from Kerrouchen in the High Atlas. The orange banding and the yellow and brown plumes are typical for that location. The orange bands and the plumes are almost opaque, while the central gray-blue portion is translucent.


Namibia


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Blue  lace agate is sometimes called banded chalcedony, which is true, but calling it "banded chalcedony" is nothing but saying it is an agate. White layers alternate with translucent layers that look blue. Like in blue chalcedony, the color is caused by Rayleigh scattering of light on tiny particles: while blue chalcedony appears blue in incident light, it will appear in a more pink tone in shining-though light.

Good quality lace agate is found in southern Namibia, in the area south-west of Karasburg, in particular at the farm Ysterputz. The image shows a typical tumbled piece.


Poland

Poland has for some time been the sources of excellent agates and is about to become one of the classical agate countries. I only own a few agates from there, and if you want to get a better impression, you can check out the website www.agates.eu, which presents agates from the Sudetes in southern Poland and bordering Czech Republic.


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 Płucki [Q097]]



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A  thunderegg from a porhyric rhyolite rock bearing a colorful fortification agate from Nowy Kósciół, in south-western Poland.



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 Sokołowiec, Poland [Q110]


U.S.A.


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A  typical thunderegg from the Blue Beds of the Richardson Ranch in the Ochoco Mountains in Oregon.



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An  agatized red horn coral, Caninia contorta, from Woodland, east of Salt Lake City, Utah. The coral is 345 million years old and has once been buried under a layer of volcanic ash that provided the silica for the little agates that fill out the voids in the coral skeleton. Meanwhile the former calcareous skeleton has been entirely replaced by chalcedony.



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This  is a cavernous vein agate that occured together with silica sinter at an old dump of the Homestake gold mine, California, south of Clear Lake. This agate has not formed in a geode, but in a medium to low temperature environment in a hydrothermal vein. Its low temperature can be concluded from the presence of cinnabar in the myrickite that also occured at the same location.



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This  agate is from a pebble plain in the desert west of Thumb Peak, Palo Verde, Imperial County, California, not far from Coon Hollow.



 

Footnotes

1 I am not talking about "agate" not being a valid mineral name. No quartz variety name counts as a valid mineral name - the valid mineral name for all varieties is "quartz". I′m talking about a specimen being a mineral or not, and that depends on its homogeneity in terms of crystal structure and chemical composition.

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last modified: Saturday, 06-Oct-2007 22:32:05 CEST
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