Chalcedony

 

last modified: Tuesday, 27-Apr-2010 22:26:23 CEST

Document status: incomplete, some images

The typical chalcedony specimen would be described as:
A dense, translucent material of white, gray or bluish color and homogeneous appearance, waxy luster, and botryoidal habit.

But "chalcedony" is a term that has different meanings depending on the context:

Here, I use the last definition.

 

Specific Properties

Chalcedony is a dense, more or less translucent, but never transparent and never opaque material. Pure chalcedony appears homogeneous and is white, gray or blue. When illuminated from the back, it may look slightly red. The blue and red tones found in pure chalcedony are caused by Rayleigh scattering of light on tiny particles, the mechanism that is also mostly[1] responsible for the blue color of the sky. More often, chalcedony contains inclusions of various minerals, which, if colorful, will taint the chalcedony. The cryptocrystalline varieties carnelian, chrysoprase, plasma and sard are all essentially chalcedony with different types of inclusions.

Chalcedony may completely fill out cavities in rocks. More often one finds layers of chalcedony on rocks, showing a warty or smooth, so-called botryoidal surface that reveals the formation from a gel. Other forms are thin stalactites and thin, rounded aggregates, so-called chalcedony roses. Pseudomorphs of chalcedony after water-soluble minerals, like carbonates and sulfates, are not uncommon. Very often the surface of chalcedony is covered by tiny sparkling quartz crystals.


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Chalcedony sometimes shows fluorescence in short wave ultraviolet light, often of green color. Pajcha Pata southeast of Tarata, Esteban Arce Province, Cochabamba Department, Bolivia. Collection Alfredo Petrov.


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Occurrence

Chalcedony forms from watery silica gels at relatively low temperatures. The silica is often released by the weathering of rocks that are initially void of silica, for example basalt, and accordingly the formation of chalcedony took place very near to the surface. Chalcedony can be found in weathering volcanic rocks, but also in sedimentary ones, often together with agate. In igneous or metamorphic rocks chalcedony is very rare and only forms veins in cracks that have been percolated by warm rising silica-rich brines. Occasionally chalcedony is found as a petrifying agent in fossils.


 

Locations and Specimen


Brazil


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A large white chalcedony rose as it can be found attached to volcanic rock walls at various places all over the world. This one is from Irai, Rio Grande Do Sul, Brazil.

At a first glimpse the thin white "stalactites" look like gel-like structures, but in fact these are thin hollow tubes. A few wider hollow tubes and their openings can be seen in the lower part of the specimen. Their presence could be interpreted as an indication of flowing watery solutions, perhaps coming out of the pores of the rock wall. Examples of chalcedony roses sitting inside their volcanic host rocks can be seen below at the U.S.A. section.



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This  is one of the ever popular ocos that are typically outlined by clear quartz crystals. They are sometimes 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.


France


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Botryoidal chalcedony with a shiny surface outlines a lithophysa that possibly formed in some volcanic ash deposit (note the pentagonal shape), but I am not sure about that. The brown, translucent chalcedony is covered by a thin surface layer of white and deep blue color. From the Departement Charente, Poitou-Charentes.


Germany


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Chalcedony  pseudomorph after coral, Offenstetten near Abensberg, Bavaria. Abensberg is the location of a stone age flint mining area, but I don't know if the specimen was retrieved from the same Jurassic limestone the flint came from.


India


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Bizarre looking blue chalcedony from a geode in volcanic rock from Aurangabad, Maharashtra. The tiny white spots are needly balls of okenite, a zeolite mineral that forms at low temperatures.


Italy


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An  irregular chalcedony geode weathered out of volcanic trachyte tuff found near Ploaghe, east of Sassari, Sardegna.



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This  gray chalcedony from Sardegna has an interesting surface structure. It looks as if silica gels had flown down the spheres and solidified into small stalactites. The label of the specimen says "Alchiro", and the dealer assured me that he had purchased it from a Sardinian trader, but to my knowledge there is no such location. The name of the city of Alghero at the northwestern coast comes closest in sound, but that is a guess, of course.



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A thin translucent chalcedony cover has grown on calcite crystals that have already been partially dissolved, forming a perimorph of chalcedony after calcite. Found near Masulas, Sardegna.


Malawi


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Chalcedony from the Chikwawa District is renown for its blue color. The surface of the chalcedony may be botryoidal or maybe overgrown with small quartz crystals, like in this specimen.


Mexico


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These  are two images of a colorful Mexican chalcedony rose, sometimes called a concha or a snake agate. Although I do not know its exact location, it very likely came from weathered volcanic rocks, judging from its similarity to specimen from the southwestern United States. This one is much more colorful, though.

One side - shown in the first image - shows a structure that indicates a gel-like origin, with a waxy appearance. The second image shows the flip-side that is typically covered with sparkling, radially grown quartz crystals. The specimen is stained by colloidal iron oxides embedded in the chalcedony.


Morocco


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Chalcedony  pseudo-stalactites, Morocco.



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Chalcedony frequently forms pseudo- and perimorphs after of other minerals. This specimen is a pseudomorph of chalcedony after a mineral with elongated platy crystals, possibly barite, in a geode from Sidi Rahhal, El Kelaâ des Sraghna.


U.S.A.


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Botryoidal  chalcedony in its typical blue-gray color from the Winnemucca Area, Lovelock, Nevada. The blue color is caused by Rayleigh scattering of light on tiny particles.



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This  is so called coral agate, chalcedony pseudomorph after coral, from the Tampa Bay, Florida. Technically it is not agate, but chalcedony showing no signs of banding. There are also coral agates with beautiful banding, one is shown in the chapter agate. The fossil is probably from the Pliocene or lower Miocene age, so it is about 5-10 million years old.

Below you see the interior of the other half of the specimen (this is a scan, not a photo, so the distant internal surface is a bit blurred). The chalcedony is dark brown but translucent with no banding and has a botryoidal surface. At the lower right end there is a casting of a shell that was embedded into the former coral reef material.

You can find a scanned copy of excerpts from the book "New and Little Known Corals of the Tampa Formation" by Norman Weisbord at www.fossilcoral.com.



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Blue-gray  chalcedony in a septaria with quartz crystals from Utah, very likely from the Dugway geode beds in Juab County. This piece is on the border-line between agate and chalcedony, as it shows a faint banding. If it was more colorful it would probably be called agate. Dugway geodes are known for prismatic rock crystals that occasionally occur in small groups or alone.


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If you want to look for chalcedony roses, the Turtle Mountains in Southern California are one of the best places to go. Chalcedony roses can be found at many places in the deserts of the South-Western U.S.A. and in neighboring Mexico, in washes, on desert planes, or in the host rock.

I have tried my luck in the Mopah Range area around Mopah Spring, an oasis. Fig. 1 is a view of the oasis, surrounded by palm trees, and Mopah Peak, the remnant of a volcano that formed in the Miocene age during the extension of the Basin and Range area of Southwestern U.S.A.. Fig. 2 shows a late afternoon view from a different perspective. Chalcedony, chalcedony roses, and sometimes rock crystals can be found weathered out on the ground or in the walls of ancient lava flows, like the ones that form steps on the right side in Fig. 1. They apparently cannot be found in the pyroclastic conglomerates and tuffs that form bizarre towers like the ones on Fig. 2.



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This  image shows an aggregation of chalcedony roses in host rock. The rock surrounding the roses has been attacked by the percolating waters and shows signs of dissolution, with a sponge-like appearance and a crumbly consistence. The yellow dots on the left side are lichens. I wish the image had a better quality, but it was already getting dark.



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White and pink chalcedony roses from the Mopah Range in the Turtle Mountains. These roses have weathered out of the volcanic host rock and could just be picked up undamaged from the ground. They are also named conchas. The "backs" of the roses usually have the typical waxy surface of chalcedony, but the frontal side shows radially grown quartz crystals on its edges and sometimes an icing of tiny sparkling quartz crystals in the center.


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Some  only show the typical chalcedony surface with its waxy luster, like the second, "mouth-shaped" specimen with a nice pink color.


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Many  of them have bizarre shapes like the third specimen with a "cup" inside a cavity that is outlined with small quartz crystals. The cup is actually a funnel that has a little tube on its bottom connecting it with the other side. Such hollow tubes are not uncommon on chalcedony roses, and a specimen from Brazil shown above shows several of them.


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This  picture shows a pink chalcedony rose with waxy luster in its host rock.



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This  white and translucent chalcedony rose has been retrieved from from a wall of weathered lava at the Mopah Range in the Turtle Mountains, California.
Such specimen are remarkable for their shape: most chalcedony roses are flat, but these are really formed like large flowers.

The location of these chalcedony roses was just 2 kilometers away from Mopah Peak. The formation of these chalcedony roses was obviously a secondary process that took place long after the lava has solidified and cooled, and is linked to the circulation of watery solutions in large cracks inside the rock, as there are no signs of chalcedony or agate within the small cavities of the rock.


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This  picture shows a specimen on matrix, a volcanic lava with many small, elliptic, and mostly empty gas cavities. Note the wreath of platy quartz crystals enclosing the rose. On the right side there is some of the glossy botryoidal chalcedony that has been overgrown by quartz crystals elsewhere.


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Chalcedony roses and agate can be found at many gravel planes near old volcanoes in the Southwestern United States, as the one shown in Fig. 3. It is a southward view of the Mule Mountains with the prominent Thumb Peak, a remnant of an old volcano like Mopah Peak, in Southern California. The whole area is well known for nice agates that - with some patience - can even be picked up from the ground. The agate shown on the introductory page is from this location.

Fig. 4 is another view of Thumb Peak, from the South. Note the many dykes running through the foothills. The two chalcedony specimen in the next three images have been found at the rightmost hill. Because of the good sight, it all looks very small, but Thumb Peak rises a few hundred meters above the surrounding plane. This spot is only accessible by feet, and the pictures have been taken in a very rainy El Nino year in early April, that's why it is so "green". It was only a bit above 30°C, but I was almost running out of water. In the summer a hike might turn into a one-way trip.



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Brown  botryoidal chalcedony from a spot about 2 km south of Thumb Peak in the Mule Mountains, California. To the right there are some flat quartz crystals. Below is another image of the same specimen, showing how chalcedony has filled out the cavities in the volcanic debris.



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Another  specimen from the same location in the Mule Mountains, with (from left to right) botryoidal gray chalcedony, radially arranged parallel grown quartz crystals, botryoidal chalcedony with a coating of small quartz crystals, and brown to black chert-like chalcedony.



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Very  irregularly grown chalcedony in a small gas cavity in basalt from the Opal Hill Mine, Palo Verde, California, on the east side of the Mule Mountains.



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Cauliflower-like chalcedony, covered by small quartz crystals in a small geode from the so-called "Potato Patch" in the Hauser Beds, Imperial County, California.



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This  is an example of chalcedony found in sedimentary rocks. It was found at the trail down to the Colorado river at Grand Canyon, Arizona. The chalcedony is covered by tiny sparkling quartz crystals.



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Fire agate found at the Opal Hill Mine, Mule Mountains, west of Palo Verde, Imperial County, California. Fire "agates" (they are not really agates) are only found at a few spots in California, Arizona, and Mexico. They contain limonite inclusions of the kind that also gives quartz crystals a shiny orange surface. (These inclusions initially covered the botryoidal surface of colorless chalcedony, and are later overgrown by other layers of chalcedony.) When tumbled and polished, they show a nice and vivid play of orange, yellow, and green colors.




 

Further Information, Literature, Links

Footnotes

1 The blue color of oxygen does also play a role.

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