Agate

 

last modified: Wednesday, 22-Aug-2012 23:54:14 CEST

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It  is not easy to explain the term "agate". Very often agate is simply defined as "banded chalcedony", and for most practical purposes this should be sufficient.

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:

An "ideal agate" by that understanding would be 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 properties that are only found in agates. There are other types of chalcedony that are clearly and sharply banded, but should nevertheless not be called agates. The "look" of specimens can be very misleading. To really understand agates one has to look at the microscopic structure of agates and other types of chalcedony, and that literally means that one has to use a microscope.


Strictly spoken, an 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

Unaltered agate is a tough, dense material with dull to waxy luster and conchoidal fracture. It is by far less brittle than rock crystal. Agate is somewhat porous and usually contains small amounts of water (0.5% to 1.5%, see discussion on mineral composition below).

Color

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 . The color is caused by various embedded minerals, of which iron oxides and hydroxides are most common, giving yellow, brown and red colors. A "pure" agate is white, gray or blue-gray.

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.

Banding

The banding is the most characteristic visible 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 commonly 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.


Initial wall-lining band

When one compares the banding patterns of many agate specimens, one will notice that in practically all agates the outmost layer that covers the wall of the cavity looks different. In most cases it is more translucent and appears darker, and often it is less pigmented by embedded minerals. This band is normally thin and often does not show any finer banding. It is even present in many Uruguay agates that otherwise lack any wall-lining banding. The structural peculiarities of these bands indicate that the agate formation is not a continuous process, but involves several steps.


Fibers 


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An important, yet well hidden feature of agates is their fibrous structure. Agates are made of microscopic quartz crystals, but these are not oriented randomly. They are arranged in groups of roughly parallely oriented crystals, and these groups form "fibers". This is not a unique feature of agates, the fibrous structure is also found in botryoidal chalcedony.

Under normal circumstances these fibers are invisible, as they are all made of the same material and the optical properties of the fibers are almost identical. But in very thin and colorless slices the fibrous nature of the agate gets revealed to the unaided eye. The first image to the right shows such a slice of an agate geode from Brazil in incident light. The slice is just 3 mm thick and only shows a very faint banding with little color.

When the slice is illuminated with a point-like light source from behind (second photo), one can see another pattern superimposed on the banding: fine lines emanate from the rim of the geode to its center. The lines run perpendicular to the agate banding and often extend across several bands. It is important to note that the lines emanate from the wall and not from the center, this can best be seen in the left corner of the specimen where several groups of lines radiate from certain spots at the wall. The pattern is rather strong in the periphery, but it is also present close to the center made of quartz crystals. When the slice is tilted slowly, one can observe a glimmering movement in the pattern, and there is also a faint play of colors at some places.

This line pattern is caused by fibers that are roughly aligned in parallel and that run from the geode wall towards the center of the geode. One should not think of fibers as individual threads that make up the fabric of the agate. The fibers are not separable from each other, one cannot pull an "agate fiber" out of an agate - the agate fabric is very dense and the quartz crystals of neighboring fibers are tightly interlocked and not separated by a large gap. The fibrous consistence of agate merely reflects the oriented intergrowth of the quartz grains. Birefringence and refraction of quartz crystals depend on the angle of the light passing through and these subtle differences are sufficient to see the line pattern. As shown in electron microscopic studies (Monroe, 1964), small voids between the grains preferably line up parallel to the fibers and also play a role in making them visible. The fibers are also present in other types of chalcedony, so it makes more sense to call them "chalcedony fibers", and not "agate fibers".

However, the best way to visualize the fibers is to use polarized light. This is the first encounter with polarized light in this chapter, and I'd like to use this opportunity for a rather personal remark.

Agates have been structurally characterized by their behavior in polarized light.
Only recently these characterizations have been complemented, but not superseded, by electron microscopic studies. The results of studies of chemical and mineral composition are all viewed in relation to the structural features found in optical microscopy.
It is not possible to understand what characterizes an agate and if and how it differs from other cryptocrystalline quartz varieties without studying agate thin sections with a polarizing microscope. Unfortunately, despite their immense educational value one hardly ever sees photos of agate thin sections in collectors' magazines.

The physics underlying the method is rather difficult to understand and will be explained in a chapter of its own, here I can only give a brief explanation.
When two polarizers (short for polarizing filters) get placed in a row, and when the polarization planes of the two filters are perpendicular to each other (when the polarizers are "crossed"), no light can get through both polarizers. But when a birefringent substance, like a lot of minerals, is placed between the two polarizing filters, it interacts with the light, resulting in interference colors. The interference colors are characteristic for the mineral and can be used to identify the minerals and determine the relative orientation of the crystals with respect to the plane of polarization. The key message to memorize here is:
The observed color depends on the angle of the crystal relative to the polarization plane.

The interference colors are usually studied in so called thin sections, very thin slices of minerals and rocks glued onto a glass slide for microscopy. The most common thickness used in rock thin sections of all sorts is 30μm. This is very thin, of course. One can try to use thicker slices, but the interpretation of the observed colors gets more and more difficult, and many rocks and minerals have to be very thin to allow enough light through.


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This is the agate from Ashland, Oregon, that has already been shown above. It had the right size to fit on a object slide, so it was chosen as a source for a thin section. This photo shows the agate in incident light.

Below are two photos of a thin section of that agate. The outline shape of the agate is not quite the same, because some of the frontal material had to be removed from the specimen during the preparation of the thin section. These photos were done with the same setup and camera as the first shot, not with a microscope, the difference is as follows: 1. The agate was illuminated from the back; 2. There was a linear polarizing filter behind the agate; 3. there was a linear polarizing filter on the lens of the camera. Both polarizers are "crossed", that is, the polarization planes are perpendicular to each other.

The pattern seen in the thin section with crossed polarizers is strikingly different from that in incident, unpolarized light. It is made of white, gray, and black lines, which, just as in the specimen shown above, emanate radially from a few spots at the wall. Areas with lines running roughly horizontal and vertical look darker in the first photo, which at some places gives the pattern a pinwheel-like look, in particular in the left and right corner. This is largely depending on the orientation of the polarizers: the second photo shows the agate with the polarizers rotated counter-clockwise by about 30 degrees, and the "pinwheels" did rotate by 30 degrees, as well.

Groups of lines that emanate from a common point form polygons with straight boundaries. These boundaries are real structural features and do not move when the polarizers are rotated.

At the bottom of the agate you can see an interesting feature: the banding pattern is not symmetric and the fibers are not perfectly straight here. They have apparently been bent towards the oldest wall lining bands that lie very close to the bottom, and not at the center as usual.

I will not give a full explanation of the physics of the observed pattern here, as it is far to complex to be put into a single paragraph (the behavior of quartz in polarized light will be discussed in a chapter of its own). But the bottom line is this: the color depends on the orientation of the quartz crystals in the agate and areas of similar color (or brightness) are made of crystals with the same orientation. Hence, when we see a line running through the structure, it must be made of crystals of equal orientations. And when we see that the overall brightness changes gradually, this is so because the average orientation of the crystals in the agate changes gradually.

So agate is called a fibrous chalcedony because it is apparently made of fibers that consist of chains of quartz crystals of similar orientation.


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The next photo shows a close-up of the thin section. The fibers are a bit easier to see, although one can also see that some areas look more "grainy", with no long fibers. In these areas the fibers have been cut obliquely, so the line pattern cannot be fully seen. One can also see that the banding pattern is faintly superimposed on the fibrous pattern, and that - like in the specimen shown above - the fibers run perpendicular to the banding pattern. When one looks at the geometry of the fibers and the overlying banding, the polygons appear to be sectors of spheres, and the agate bands form arcs that seem to be parts of concentric rings.

Another thing can be seen more clearly on that photo: the outer rim looks very different. It is also made of black-and white lines, but the pattern is much more coarse. This is the initial wall-lining band of the agate. In the photo of the agate in incident light it can be seen as a dark rim that is more translucent than the rest of the agate.


Spherulites and Wall-lining Banding

The patterns observed in transmitted and in polarized light, namely that agate seems to consist of a mosaic of spherical sectors with straight boundaries, that fibers radiate from a common point at the geode wall and that the bands are composed of arcs, can be interpreted to reflect the spherulitic nature of agates.


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Spherulites are spherical ("ball-shaped") bodies or patterns that develop by radial growth. Spherulitic growth is not uncommon in nature. For example, spherulites form during so-called devitrification processes ("de-glassing" processes) in obsidan, a natural glass. During devitrification the glass slowly crystallizes to minerals like cristobalite and feldspars. The photo shows a close-up of a partially devitrified black obsidian from the island Lipari, Italy, with lots of small spherulites in it. It has been estimated that such structures grow within days to weeks (Watkins et al., 2009).


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Figure 1.1 demonstrates what happens if spherulites continue to grow freely until they occupy the entire available space. These sperulites have been drawn to consist of fibers that grow radially from single nucleation points, the starting points of crystallization, at some random spots in a solution. They remain perfectly spherical as long as they don't get in touch with each other (a).

Changes in growth speed or in the type of included impurities will cause changes in the structure of the crystals at the "growth front" on the surface of the spheres, like the size of the crystallites or the transparency of the material. This will, of course, affect all growing crystals in all spherulites, and accordingly spherulites that started growth simultaneously will develop the same structural patterns, or put simply, look very similar.

When they continue to grow, the spherulites will simply fill out the entire space (b, c). An interesting feature of such a rock made of spherulites is difficult to see, unless you color the areas that belong to individual spherulites, as it has been done in d: The boundaries between the spherulites are straight lines, and the spherulites assume a polygonal shape.


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If radial growth does not start at nucleation points that hover freely in a solution like in Fig.1.1, but starts at an already present boundary, like a rock wall, the texture that develops looks different, as shown in Figure 1.2. The area occupied by the crystals that start from a single nucleation point assumes a near-rectangular shape, again with straight boundaries to the neighboring groups of crystals. One can also see that the concentring rings around the nucleation points merge into continuous banding patterns. The bands get more and more straightend out at larger distances.


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If the nucleation points are distributed along the wall of a cavity, the typical wall-lining banding develops. Depending on the density of the initial nucleation points on the wall, the banding pattern will look more jagged like in so-called "fortification agates" (left column in Fig.1.3) or will follow the outline of the cavity more uniformly (right column in Fig.1.3). I have marked the different developmental stages of the spherulites with different colors to emphasize the geometry of the resulting banding. It is remarkeble how much the banding in these figures, which have been drawn following the simple rules in Fig.1.1, resemble natural agate banding patterns.

It should be noted, though, that the initial wall-lining band often does not show obvious signs of spherulite growth.



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To see that this is not all theory, a real world example: a small agate from Waldhambach near Landau, Rheinland-Pfalz, Germany. It shows numerous nice circular spherulites along the outer red layer. The agate is translucent, so one can see that the sperulites are ball-shaped. In particular in the upper part of the agate one can clearly see how the spherulites slowly merge to form a continuous banding. Collection, photo and copyright Klaus Stubenrauch.


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When the fiber growth pauses and resumes after a while, growth may continue from new nucleation points on the surface of the last layer. So what is shown in Fig.1.3 can occur several times in an agate.

The two photos to the right show an example of this, a small elongated agate nodule with a core of quartz crystals, and parts of the outer rim made of yellow-green calcite crystals. Most of the chalcedony in the flesh-colored agate between the calcite and the quartz core has been replaced by calcite, but the agate banding pattern has been preserved very well.

The lower photo is a close-up that shows the different generations of spherulites that merge to form continuous bands during growth. The banding patterns of well separated spherulites resemble each other as long as the spherulites belong to the same "generatin" and grew at the same time. In the upper third you can see three spherulites, sitting like snowballs on the last agate layer before they got overgrown by colorless quartz crystals.
The agate is from the Juchem quarry, Niederwörresbach, north-east of Idar-Oberstein, Germany.


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It should now be much easier to interpret certain phenomena in agates, like the one shown on photo to the right. It's simply a slice of a colorless agate nodule from Brazil that has been cut very close to the rim, and not through the center, as usual. When it is illuminated from the back, one can observe a strange cloud-like pattern. What you see are simply cross-cut radial fiber bundles in the sectors of intergrown spherulites.



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One should not be too obsessed with the idea that wall-lining banding has to be "wall-lining", in fact this type of banding can develop along all kinds of structures. The banding in this agate developed mostly symmetrically on both sides of the long yellow and greenish "threads", but also along orange-red structures in the upper center. These incusions obviously predate the agate banding, they are not a secondary filling of cracks. This can best be seen in the lower left corner.
The specimen is from Gyöngyöstarján, Mátra Mountains, Heves County, Hungary, a locality known for bizarre, irregular banding patterns with colorful inclusions. Agate from this locality is commonly associated with botryoidal chalcedony, as seen on the upper side of this specimen.


So far the spherulitic agate fibers have been dealt with as if they all had a very similar look and structure, but in fact a large variety of wall-lining banding patterns can be observed under polarized light.


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A specimen from Kerrouchen, Morocco, serves as an example. The specimen is rather small, so almost the entire specimen could be used to prepare a thin section. But before discussing the thin section, it is worth to look at certain structures that are already visible in the full resolution photograph (4448x3200 link).

The upper part shows the typical wall-lining banding, with a strong contrast between the outer, hematite colored bands, highly translucent bands, and white bands. The white and red bands get thinner to the left, this is not so obvious for the translucent bands. In the white bands one can see a sign of a fibrous structure: it looks like a fine furr that has grown on the underlying translucent band. At the center of the agate one can see fine triangular ripples, here several generations of small druzy quartz crystals have overgrown each other.

The lower yellow and brown part of the specimen looks a bit more chaotic. Directly under the agate one can see a complex pattern that looks "fractal". These and similar looking patterns are called plume agate. The general direction of growth seem to be from the bottom to the top, but although it is composed of consecutive layers, it lacks continuous banding, with the exception of the outmost layer. Some of th eplume agate can alos be seen at the upper rim of the specimen. This structure obviously predates the agate that developed along its surface. It is interesting to note that the plume agate for the most part is well separated from the wall-lining agate by a thin dark line that resembles the typical initial band in agate geodes.

At the bottom one can see a few partially merged dots with yellow core and brown rim along cracks as well as straight yellow lines outlining thin cracks. The dots seem to be spherulites, but there's no hint of banding and there is not much to say about the nature of the yellow material except that it is harder than the brown host rock that has been ground out more deeply.


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When the thin section is placed between crossed polarizers, the spherulitic nature of the upper part gets revealed. However, compared to the Oregon specimen shown above, the pattern is much more complex, and the banded nature of the agate is also more obvious.

The part with the plume agate looks as complex as in the first photo, the material is chalcedony, but it does not show the same level of order as the wall-lining agate. And despite some indication of consecutive deposition of material, there is no fine-structured repetitive banding visible, at least at that level of magnification.

The homogeneous yellow dots at the bottom part that lack banding turn out to be spherulites made of fibrous chalcedony, presumably stained by iron hydroxides. But different from those found in wall-lining agate, these spherulites apparently lack banding.


Horizontal Banding


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The geometry of horizontal banding suggests a nature and origin that is different from wall-lining banding, and on a macroscopic level it appears not to be made of spherulites. Material has obviously settled slowly or "sedimented" on the base of the agate by gravitation and solidified. Horizontal banding is often used as a "frozen spirit level" to determine the original orientation of the agate. It is difficult to come up with a better explanation for the pattern than sedimentation of particles. In fact, agates have been found in which the orientation of the horizontal banding changes suddenly - this is interpreted as a result of tectonic events.
Horizontal banding is very common in thundereggs that are mostly found in volcanic rocks of acidic composition like rhyolite, in particular in rocks with a large amount of glassy components. The photo shows a typical thunderegg from the Blue Beds of the Richardson Ranch in the Ochoco Mountains in Oregon.



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To demonstrate the structural differences I've chosen an agate slice from Brazil that shows both types of banding.

The lower left corner of the agate slice shown in the upper photo has been cut out to prepare a thin section. The rather faint banding pattern looks much more defined in polarized light with crossed polarizers, and the wall-lining agate bands show the typical spherulitic growth that extends over several bands.

The horizontal banding looks different. Each layer seems to have developed independently from the others, there are apparently no structures that extend into neighboring layers. A few layers, like the dark central one, show spherulitic growth with typical wall-lining banding patterns and appear to be a continuation of a wall-lining band. Overall the look of the horizontal bands is more "grainy", with large variations in apparent grain size. At the bottom one can see large, randomly intergrown spherulites which lack internal banding. A larger magnification would reveal that most of the horizontal bands are made of such spherulites, albeit much smaller ones. In the upper part the slightly curved and darker bands are actually wall-lining bands that have grown on the last horizontal band.



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 Kardzali, Bulgaria


Iris Banding


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Thin slices of very translucent agates may exhibit a vivid play of spectral colors when viewed in transmitted light. The photo shows such a slice of an agate geode from Brazil: on top the agate is seen in incident light and only shows a very faint banding pattern with only the outer parts showing a little color. The lower view shows the agate illuminated by a distant point-like light source from behind (in even illumination the agate will just look milky). 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 grating is made of alternate layers of slightly higher and lower refractive indices that run parallel to the regular bands. The observed colors are interference colors that depend on the spacing of the banding, the angle of the transmitted light and that also 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 specimen shown in the photo is 3mm thick.

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.

Taijing and Sunagawa (1994) found that, within an agate, iris banding is commonly seen in areas close to the transition to drusy, macrocrystalline quartz, which they interpreted as an indication that it develops at late periods of the agate formation. One has to keep in mind, though, that because of the geometry of the banding patterns, an agate is more likely to be cut in a way that is favorable for iris banding to be visible close to the center than it is at the rim.

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").


Fluorescence


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Like chalcedony, agates from many localities show a fluorescence in short-wave ultraviolet light, sometimes also in long-wave ultraviolet light. The most common fluorescence color is green, other colors are orange or blue, but they are much rarer. Many agates are colored by embedded iron compounds, but Fe3+ quenches the response to ultraviolet light, so these agates do not show fluorescence.

The two photos to the right show a thin agate slice from Brazil, with an initial band, wall-lining-banding, and Uruguay banding. The first photo shows the agate in incident light, the second photo in short-wave ultraviolet light. The green fluorescence pattern reflects the banding, the central milky patch shows a more unusual blue fluorescence.



Mineral 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; Götze et al., 1998). 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 the water 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.

Enhydros 

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. The presence of water in the geode is sometimes interpreted as an indication of an agate formation in a watery environment, but of course the water could just as well have entered the geode later.

Polygonal Agate 


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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.



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A typical example of agates from Wendelsheim, west of Alzey, Rheinland-Pfalz. The agate has filled the voids between platy crystals that have later been dissolved and replaced by chalcedony and small quartz crystals. In a sense, this is the miniature version of polygonal agates.


 

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. The material is a mixture of length-fast chalcedony and quartzine. They usually do not show any agate banding as discussed on the agate page, so in my opinion , these do not qualify as agates. In lapidary terminology, these are agates, though, and that's why the specimen is presented in both the agate and in the chalcedony sections.


Czech Republic


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A typical agate from Horní Halže, Krušné Hory Mountains, Bohemia. One can see three interesting patterns, rounded triangles, one to the top, a large one with a bright center to the left, and a small one to the lower right. The angles and shapes of the three triangles are very similar and look much like cross sections of scalenohedral calcite. The pattern might have developed around perimorphs of calcite crystals that were completely dissolved.
Collection Jacek Szczerba.


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 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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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.



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 This specimen with two agates in one lithophysa from Sankt Egidien, Sachsen (Saxony), Germany, has already been shown above, in the discussion on agate formation.

It is shown again together with a close-up on the upper agate simply because it has very beautiful spherulites and little "UFO" inclusions made of hematite. One can recognize radial chalcedony fibers in some of the lower spherulites (difficult to see in the down-scaled image).


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.



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Concentric banding developed around thin tubes and tiny grains in the peripheral part of this agate from Aouli, northeast of Midelt.
Collection Jacek Szczerba.


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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Another example from Plóczki Górne, much larger than the previous one, and with a very nice wall-lining banding.
Collection Jacek Szczerba.



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Another agate from Plóczki Górne, showing a similar succession of plume agate and fortification banding as the specimen in the previous photo.
Collection Jacek Szczerba.



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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.



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A typical specimen from the locality Różana, very close to the thundegg locality at Nowy Kósciół, Lower Silesia. Here the agates are not found as thundereggs, but as small, rounded nodules in a massive and dark volcanic host rock of probably basaltic composition.


U.S.A.


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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 central part of the former calcareous skeleton has been entirely replaced by chalcedony, while the colorless outer parts are made of calcite.



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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.




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 Potato Patch, Hauser Agate Beds, Imperial County, California.



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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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