Agate

 

last modified: Saturday, 15-May-2010 18:03:54 CEST

Document status: construction site


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 exhibits some colored pattern or shows banding.


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So despite being just gray, the specimen in the image to the right from Ashland, Oregon, shows banding and would qualify as an agate.

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. Currently I cannot tell if the structural characteristics of the forementioned "moss agates" are so different that one should rather not call them agates.

Strictly spoken, agate is not a mineral [1]. Agate does not have a homogeneous 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. The color is caused by various embedded minerals, of which iron oxides and hydroxides are most common, giving yellow, brown and red colors.

Agate has long been known as a porous material that can be dyed easily and numerous methods to change, enhance or add color have been developed. Specimen that lack vivid colors and banding, like many agates from Brazil, are cut into thin slices and artificially "enhanced" with various dyes, yielding deep green, blue, pink, and sometimes more unsuspicious brown tones.

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 is nothing special about enhydros except for being quite rare, they simply did not dry out yet, like all the other agates did. Remember that agates form in a watery environment.


Banding

The banding is the most characteristic property of agates. It is mostly a result of 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. In old agates that have been subject to weathering and chemical alteration the differences in translucency may disappear.

The thickness of the individual layers varies greatly in different agate specimen. In the finely banded agate from Agate Creek shown in the image below one can count more than 200 layers per centimeter. Other specimen show a much lower density of layers.

Two types of banding are distinguished in agates:


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In this image wall-lining banding (left) is contrasted with horizontal Uruguay-type banding (right). It is a detailed view of the specimen from Agate Creek, Queensland, Australia, that is also shown under Locations.

In this section of the specimen the wall-lining layers are homogeneously colored and spaced quite evently. By contrast, the horizontal layers look much more diverse: some layers appear granular and less translucent, one can recognize two areas made of quartz crystals (notice the bright reflections), one layer is colored orange, and in the upper part there is an area with ripple banding. The most interesting and tale-telling feature of horizontal bands are the numerous little particles that apparently have precipitated on some of the layers.

Some, but not all horizontal layers merge with wall-lining layers. So at least some of them did develop independently from the wall-lining banding.


Iris Banding

Thin slices of clear agates may exhibit a vivid play of spectral colors when viewed in transmitted light. The colors are caused by a very fine banding that is superimposed on the regular, wall-lining agate banding, and that acts as a diffraction grating on the light (Jones, 1952). The observed colors are interference colors that change with the viewing angle. The strongest effect is seen in sections of less than 1mm thickness that were cut perpendicular to the banding (just as most agates are cut to show their regular banding). The grating is made of alternate layers of slightly higher and lower refractive indices that run parallel to the regular bands (Jones, 1952).

Agates showing interference colors are sometimes called "iris agate" or "rainbow agates", as the iris banding is considered as a special phenomenon that is only found in certain types of agates of a few locations. Studies on agates from different locations indicate that iris banding is a common phenomenon that is usually masked by strong pigmentation or low translucency of the specimen (Jones, 1952; Frondel, 1978; Heaney and Davis, 1995). Agates are also rarely cut into slices that are thin enough for the interference effects to be observed.

Within an agate, iris banding is commonly found in areas close to the transition to drusy, macrocrystalline quartz, an indication that it develops at late periods of the agate formation (Taijing & Sunagawa, 1994).

To cause strong interference effects and bright primary colors, the distance between the elements of the grating must be just a few times larger than the wavelength of light (between 250-700 nm, or 0.25-0.70 micrometers). In optical microscopy studies, Jones observed variations in the spatial frequencies of the banding between 15/mm and 600/mm, corresponding to wavelengths of 67,000 nm and 1670 nm, with the colors getting more saturated the finer the banding is. Similar spacings between the bands, between below 1 and 100 micrometers, were observed by Frondel, 1978, and Taijing and Sunagawa, 1994, with widths of 1-3 micrometers being most common. Iris banding seems to correlate with periodic variations in quartz grain size (Taijing & Sunagawa, 1994) and chemical composition (Frondel, 1978, Heaney and Davis, 1995, see under "Composition").


Composition

Like many other types of chalcedony, even "pure agate" is not necessarily made of pure quartz. It may contain varying amounts of the silica polymorph moganite, typically between 1% and about 20%, mostly around 5% (Heaney and Post, 1992). Of all silica polymorphs moganite shows most structural similarities with quartz, but it is never found in pure form. The moganite content of agates depends on their age, as the moganite slowly converts into quartz, apparently agates older than Silurian age are basically pure quartz (Moxon and Rios, 2002; Moxon, 2004).

A chemical analysis of the total composition of agates shows small amounts of water in addition to silica, typically in the range from 0.5% to 1.5% (as reviewed by Graetsch 1994). Parts of which are captured in the pores between the tiny crystal grains, while some of it is bound chemically in silanole (Si-OH) groups. This water can be partially driven out of the agate by heating to 200-500°C for a couple of hours. The loss of water is apparently in large part due to the decomposition of the silanole groups (Fukuda and Nakashima, 2008).

Individual agates show a zonation of water content (both as silanole-groups and molecular water) parallel to the banding, but often on a much finer scale, with individual compositional bands measuring around 1 micrometer (Frondel, 1982; Frondel, 1985). Submicrometer-scale compositional bands seem to coincide with iris banding (Frondel, 1978). In his studies Frondel (1985) named zones H, M, and L, corresponding to high, medium and low lewels of OH. He found that the compositional zones exhibit peculiar spatial patterns. Zones often show oscillatory variations of OH-content, leading to repetitive and symmetric patterns like "HLMLHLMLHLMLH" or "MHLHLHMHLHLHM" that extend over several millimeters.

Compositional zoning that coincides with iris banding has also been observed by Heaney and Davis, 1995, for aluminum, Al, and sodium, Na. The concentrations of both trace elements correlate, which indicates that Na is trapped in the crystal lattice as a charge compensator for trivalent Al that replaces silicon in the SiO4 groups.

Agate is often said to contain some opal, but this could not be generally confirmed in recent studies (e.g. Heaney et al., 1994). Probably impurities, like the presence of water and the formerly unknown mineral moganite, and lattice defects commonly found in chalcedony distorted the measurements of the structural properties in a way that could be interpreted as presence of some amorphous substance like opal. In a review on the nomenclature of micro- and non-crystalline silica minerals, Flörke et al., 1991, mention opal-C as a typical matrix component in horizontal, Uruguay-type bands in agate. However, in his own review on microcrystalline silica minerals, Graetsch, who is a co-author in the forementioned article, states that "Further investigations have provided no evidence for the presence of opal" (Graetsch, 1994, page 212).


Structure

Agate is made of varying amounts of different types of chalcedony and the silica polymorph moganite:

All agates contain either length-fast chalcedony or microquartz, or both.

Chalcedony, quartzine, pseudochalcedony and microquartz differ in the way their quartz crystallites are intergrown. As explained in detail in the chapter Types of Quartz, they appear to be made of fibers when viewed in polarized light in a microscope (for an overview see Braitsch 1957; Frondel 1978; Flörke et al. 1991; Graetsch 1994; Cady et al. 1998). Their fibrous look is a result of the relative orientation of the crystallites in the matrix, and since there are no real separable fibers in the chalcedony substance, I call them virtual fibers. However, when agate is artificially dyed, the color penetrates the rock much more quickly in the direction parallel to the fibers, which are mostly oriented perpendicular to the banding, than along the layers (Bauer, 1904; Liesegang, 1915), so the term "fibers" is not completely inadequate. Scanning electron microscopy studies have revealed that pores in the chalcedonic substance tend to be arranged along lines parallel to the virtual fibers (Monroe, 1964). Subsequent scanning and transmission electronic microscope studies (Taijing & Sunagawa, 1994) have revealed that virtual fibers are composed of even smaller, slightly elongated crystallites (8-100 nm, invisible in optical microscopy) that are aligned on a much finer scale parallel to the orientation of the virtual fibers.

The dominant component in agate is length-fast chalcedony. Quartzine, microquartz and pseudochalcedony are present in much smaller amounts. As mentioned above under "Composition", the moganite content is highly variable, but usually low.

These components may also be found in other cryptocrystalline quartz varieties, like flint or chrysoprase, so their presence is not special to agates. What is special for agates is that these phases are intergrown in a highly ordered and characteristic manner. In a typical agate, consecutive layers of length-fast chalcedony lie parallel to the banding, with occasional layers of quartzine and areas of pseudochalcedony interspersed. The chalcedony is often intergrown with moganite.

The virtual fibers in these layers are oriented in a specific way:

The individual quartz crystallites of the virtual fibers predominately show Brazil law twinning, both in lenght-fast chalcedony and quartzine (Graetsch, 1994; Taijing & Sunagawa, 1994; Xu et al., 1998). Periodic variations of grain size along the fibers were found to occur in agate parts that show iris banding (Taijing & Sunagawa, 1994).


 

Occurrence

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

Agates can be found in sedimentary rocks. For example, there are several locations in the northern and southern Schwarzwald (Back Forest), where small agates formed in cavities inside chalcedony in a Triassic sandstone (Obenauer, 1979).


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Similar specimen are found at the other side of the Rhine graben in the northern Vosges in France, also in Triassic sandstone. The specimen on the photo is a slice of sandstone in which voids have been filled successively by 1. a creme to flesh-colored, porcelaine-like, layered material that appears to be silica sinter, 2. chalcedony with red-brown inclusions of hematite, 3. gray agate and finally 4. small druzy quartz crystals. The reddish, fine-grained sandstone has mostly lost its porosity and seems to be impregnated by silica. Silica sinter would indicate a deposition from medium-temperature brines. From Walscheid, Département Moselle, France.


Hydrothermal Veins

Agate is occasionally found as "vein" agate in and around low-temperature hydrothermal veins, for example at certain ore deposits. An interesting combination is sometimes found at the Grube Clara in the Black Forest, Germany, where thin banded layers of chalcedony formed inside cavities of massive fluorite veins.


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This  is a cavernous vein agate that occurred together with silica sinter at an old dump of the Homestake gold mine, California, south of Clear Lake. The low temperature of the environment can be concluded from the presence of cinnabar in the myrickite that also occurs at the same location. Note the presence of Uruguay-type banding.



Silicified Wood


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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.

The image shows a cross section of a silicified wood branch from an unknown location in India. Cracks in the wood have been filled with colorful agate. While the wood substance has been replaced by yellow chalcedony, probably dyed by hydrous iron oxides, the agate fillings in the voids are stained by reddish hematite. The difference in color indicate that the silicification of the wood and the agate formation took place at different times. The image below shows the core in detail. Collection, photos and copyright Klaus Stubenrauch.

 

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 surprise that there is a countless number of agate "varieties", Zenz, 2005, lists 122 different varieties, 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.

Language barriers cause more difficulties. 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.

Onyx and Sardonyx 

Onyx is simply a black-and-white agate and sardonyx a red-white and rarely red-white-black variant. There would probably be no separate name for it if there wasn't a long tradition of cutting cameos from onyx and sardonyx.

Onyx is not to be confused with onyx marble, a banded marble (consisting of calcite, not quartz) used for ornamental works, which is frequently sold as "onyx".

Except for the color, with the black parts being opaque in good specimen, there is nothing specific that cannot be found in other agates. The "ideal onyx" is made of parallel alternating layers of black and white and thus cut from agate of the Uruguay-type.

There is a long tradition of dying agates to turn them into onyx for ornamental and lapidary uses and it can be very hard to tell a real onyx from an artificially dyed one.

 

Formation

This is going to be a paragraph on agate formation.

This is a very difficult and controversial subject and it will take a long time to collect all the necessary informations to write something that really helps. Until then I can only play the "myth buster" and start to list theories and assumptions that do not match the facts.


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One of these unfounded assumptions is that the banding pattern formation is linked to fluctuations in the environment. All sorts of things are suggested: variations in the ground water level, geysers, the phase of the moon, etc. Unfortunately, agates that develop next to each other in the same rock often show a different banding. Each agate does its own thing, so to say, despite the fact that there is an influence of the environment and agates of a certain locality share a lot of properties and assume a look that is characteristic for that locality.

The photo shows a very nice example of two agates in one (!) lithophysa from a rhyolite deposit in Sankt Egidien, Sachsen (Saxony), Germany, with a striking difference in their banding pattern and color. The agate is actually a bit darker than shown, I've lightened it up to show the patterns more clearly.


 

Further Information, Literature, Links

Johann Zenz has written a very nice book, ->Agates, available in German and in English. 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.


Australia


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The most famous agate location in Australia is Agate Creek in Northern Queensland (also known as "Queensland Agates" in the U.S.A.). The agate nodules at that location orginate from various, now mostly weathered Permian volcanic rocks. Thundereggs come from the more acidic rocks, while regular agate geodes came from intermediate and basic rocks, like andesites. The area produced many different types of agates in many colors, among them unique green-yellow specimen with a fine banding, like the one shown to the right. The specimen looks yellow-orange in backlight, and olive green when illuminated from the front. It also shows Uruguay banding in the central lower parts, which is not uncommon in agates from agate creek.

Botswana


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Agates  don't have to show a symmetrical banding, and agates from Botswana are renown for showing an ecccentric 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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An  typical egg-shaped massive geode filled with a very dark agate from an unknown location in Brazil. Although there is some fine banding, it is not very intense, which is typical, as Brazilian agate often shows good translucency, but weak banding.



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An  Uruguay-type agate with its typical horizontal 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. 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.



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In the 1970s agate slices with the shapes of irregular polygons appeared on the market. They were found in great quantities in the Brazilian state of Paraíba, but currently the locality is not productive any more. They are known as polygonal agates or Paraíba agates (the second name was used in Germany). Sometimes groups of neighboring polygonal agates were found that apparently were once separated by thin platy crystals. The former crystals are now completely dissolved and replaced by clay and quartz.

The first image shows the not so common case of two halves of a polygonal agate that was not cut into many slices. The agates are usually made of a thin layer of white, gray or bluish, but hardly ever colorful agate, followed by another layer of quartz crystals that outlines a central cavity. In this specimen the quartz crystals are covered by another thin layer of chalcedony.


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The second photo shows both halves put together again. In the center you can see the cut running through the geode horizontally. The geode is bounded by perfectly plane "faces" with a polygonal outline, but the shape is asymmetric with random angles between the "faces" and thus is not related to any crystal class. This irregular shape can only be explained as a cavity bound by crystal faces of neighboring crystals that got outlined by chalcedony and quartz. The triangles and criss-cross patterns present on the surface are interpreted as negative imprints of the surface patterns of calcite crystals which enclosed the cavities that would later host the agates.


Germany

The classical agate location in Germany is the area around the small town of Idar-Oberstein in the Hunsrück Mountains in Rheinland-Pfalz (Rhineland-Palatinate). For a short overview of the mining history and the geology of the Idar-Oberstein region, see the booklet Die Edelsteinmine im Steinkaulenberg und die historische Weiherschleife in Idar-Oberstein by Bambauer et al., which is also the main source of the information on the local history on this page.

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Since the Middle Ages the local Permian volcanic rocks have been exploited for agate, amethyst and jasper. This was mostly done by manual labor, and agate at those times was much more valued than it is now. The agate deposits of that area have first been mentioned in the literature in the 14th century, and till the early 16th century a small local lapidary industry had been established. Initially the agate was only collected on fields and in open pits, later underground mining began. The most famous location is the Steinkaulenberg in Idar-Oberstein, named after the many shafts and tunnels driven into the rock[2]. The image shows the entrance to the "Barbara-Stollen"[3], one of many century-old Steinkaulenberg agate mines. One of these mines can be visited in a guided tour. Another old tunnel, the "Schürfstollen" has been reopened recently, and collectors search for agates in material from this small mine.

When immigrants from the Idar-Oberstein area discovered the large agate and amethyst deposits of Brazil and Uruguay in the early 19th century, the local mines were soon abandoned. Nevertheless the plentiful imports from South America sparked off a boom of the local lapidary arts industry in the middle of the 19th century, and today Idar-Oberstein is still an important center in mineral trade and lapidary arts.


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An outcrop of the volcanic host rock at the Steinkaulenberg, very close to the old mines. The host rock is a Permian latite-andesite lava flow that is slightly tilted. The two signs indicate the base of the lava flow to the lower left and the core of the flow to the upper right. The lava contained some gas that was dissolved under high pressure, and when the lava appeared at the surface, the gas formed bubbles that were trapped in the quickly solidifying rock. Fresh andesite is not very rich in free silica, but the rock has been chemically altered by percolating waters, and the silica released during this process accumulated in the gas cavities to form the quartz varieties agate, jasper, amethyst and smoky quartz. The alteration is most prominent at the more crumbly base of the lava flow, while the core of the flow is less affected. Both zones contain agate and amethyst nodules. The field of view is approximately 4 meters.



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An agate in a matrix of a Permian volcanic rock (probably an andesite) from the Juchem  quarry, Niederwörresbach, north east of Idar-Oberstein. This location is one of the few fee collecting sites in Germany, where a limited number of collectors are given the permission to look for agate, jasper and amethyst inside the quarry during the summer weekends.
The specimen is shown "as is", it is rough, just as it was when I cracked the rock. The brown outer layer is made of calcite, and the fine banding of the agate is well visible despite its rough surface. Note the large single spherulite in the upper part that predates the formation of the calcite layer.



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Another example from the Juchem quarry, Niederwörresbach. The specimen shows three typical properties of Juchem agates: Red-brown specks of iron oxides are scattered throughout the layers, the layers are almost opaque, and the outer parts of the agate have been altered to a white fine-grained material, possibly made of quartz. The banding of this specimen is so fine that all the layers are only visible in the full-sized image.



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A "fortification agate" from the Juchem quarry, Niederwörresbach. The central part consist of an outer layer of quartz crystals and a core made of agate. The voids between the crystals have been filled by chalcedony, too, giving that parts a cloudy look. Like in the former specimen, the outmost layers have been altered to a fine-grained white material.



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A small elongated nodule from the Juchem quarry, with a core of quartz crystals, while parts of the outer rim are made of yellow-green calcite crystals.



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A  small agate with an irregular banding that has been picked up from the fields around Rimsberg at Birkenfeld, west of Idar-Oberstein. In the lower left corners you see greenish inclusions, probably chlorite.



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This small agate from Waldhambach near Landau, Rheinland-Pfalz, shows numerous nice circular "spherulites" along the outer red layer. The agate is translucent, so one can see that the sperulites are ball-shaped. Collection, photo and copyright Klaus Stubenrauch.



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Another egg-shaped specimen from Waldhambach, demonstrating how individual spherulites merge into complete layers. Collection, photo and copyright Klaus Stubenrauch.



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A finely banded agate from Waldhambach. In the lower left corner you see a funnel-like structure with thinning and distortions in the banding pattern that has been interpreted both as an entry or exit point for material to get into or out of the agate nodule, and has been called "infiltration channel".



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Sankt Egidien, between Zwickau and Chemnitz in Sachsen, is renown for its colorful thundereggs. In this specimen, the numerous red hematite inclusions almost hide the agate banding in the outer parts. The central "cavity" is made of transparent quartz crystals, and the cobweb-like distribution of hematite reflects its precipitation in voids between these crystals.



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Thundereggs from the Lierbachtal near Oppenau, Black Forest, have a very fine grained rhyolite matrix of Permian age that looks similar to lithophysae found in limestone. Many of them develop bizarre shrinkage patterns that look very different from the typical star-shape found in thundereggs.



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A star-shaped thunderegg, also from the Lierbachtal. The agate is very translucent and on the right side one can see black, moss-like dendrites, probably made of manganese or iron oxides, that have formed along a tiny crack in the agate.



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This agate has partially been altered to calcite, and shows a very unusual radial color pattern that is perhaps not related to regular agate banding. Well-developed banding is only visible in the right part of the nodule. Juchem quarry, Niederwörresbach.


Hungary


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 Gyöngyöstarján, Mátra Mountains, Heves County.


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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A small agate nodule in which ghost-like chalcedony tubes have been embedded in the highly translucent chalcedony that later filled the voids. From Plóczki Górne, Lower Silesia, south-western Poland. This location is known for its large variety of agates.



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



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The origin of this thunderegg is Sokołowiec, very close to Nowy Kósciół, in south-western Poland. Dark-banded agates are common at that spot.


U.S.A.


20mm  
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A  typical thunderegg from the Blue Beds of the Richardson Ranch in the Ochoco Mountains in Oregon.



10mm 
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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.



20mm  
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This  agate weathered out of volcanic rocks and was found on a pebble plain in the desert not far from Coon Hollow, west of Thump Peak, the remnant of an old volcano near Palo Verde, Imperial County, California.




20mm 
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A nice coral agate, a chalcedony pseudomorph after coral, from the Tampa Bay, Florida. In this fossil, the entire coral has been replaced by chalcedony substance, not just its interior. This one shows beautiful agate banding in parts of the chalcedony, but many coral agates consists only of chalcedony with no banding, like the one that is presented in the chapter chalcedony. It is a nice demonstration that agate can form at very low temperatures: the agate formed in fossilized reefs of Miocene age very close to the surface, possibly under water, in environments that have never been exposed to great heat.


 

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.

2 Named after the patron saint of the colliers.

3 Sometimes people say that the name "Steinkaulenberg" is derived from its older name, "Galgenberg" ("gallows mountain"), but it simply is the local dialect form of "Steinkuhlenberg", which translates to "stone hollow mountain" or "stone pit mountain".

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