Rhyolite agate is agate that did not grow in a bubble in basalt but inside a ball of rhyolite, a light colored volcanic rock rich in silica. Because these balls crack open in a star shape as they cool, the agate in a cut shows no round amygdale but a jagged star with long arms. Specialists speak of lithophysae; in the trade the balls are called thundereggs, in German Donnereier, formerly also porphyry balls. One of the best known German deposits is the Teufelskanzel on the Leistberg near Oberthal in Saarland. With a Mohs hardness of 6.5 to 7 the agate is one of the hard materials, while its rhyolite shell is somewhat softer.
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How rhyolite agate forms, using the example of the Teufelskanzel
The Teufelskanzel, the Devil's Pulpit, is a rock pulpit of rhyolite that juts out of a steep wooded slope near Oberthal in the district of St. Wendel, around 30 kilometers southwest of Idar-Oberstein. It belongs to the Nohfelden rhyolite massif, and its story begins with the volcanism that shaped the Saar-Nahe Basin in the Early Permian.
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Viscous lava full of gas
Roughly 295 million years ago, in the Rotliegend period, volcanoes in the Saar-Nahe Basin repeatedly erupted lava and ash. Part of it was rhyolitic: a melt with a great deal of silica, which therefore flowed as viscous as honey and barely let its gas escape. In this melt the gas collected in bubbles, and around these bubbles fine crystal fibers began to grow outwards in a radiating pattern. This created spherical structures in the rock, the lithophysae, ranging from a few centimeters up to the size of a fist.
The ball cracks open like a star
As they cooled further, the balls contracted. Because their interior solidified last and shrank the most, it cracked open, not in a round shape but like a star, along the radiating fibers. What remained in each ball was a jagged cavity with long arms, sometimes narrow, sometimes wide, never the same twice. This shape later gives the agate its typical form.
Agate fills the star
Much later, when the rock had long since cooled underground, water with dissolved silica seeped through fine cracks into the balls. In the star-shaped cavity the silica was deposited layer by layer as chalcedony, and because each new layer follows the previous one, the bands trace every point of the star. At the Teufelskanzel they are often gray, brown and light to dark blue, more rarely yellow or red. If the silica did not reach the center, a space remained into which quartz, smoky quartz, hematite or goethite grew.
Today at the Teufelskanzel
Today the volcano has long since been eroded, and the hard rhyolite stands as a rock on the slope. As the rock weathers, the tough agate balls work loose and end up in the scree. From the outside they look like inconspicuous reddish brown nodules. Only the cut reveals the star of agate inside.
People were already digging here in the 18th century, and the stones went to Idar-Oberstein for cutting. An attempt to mine the deposit systematically in 1882 failed. Because uncontrolled digging later badly damaged the slope, digging at the Teufelskanzel has been prohibited since the early 1980s, and the ban is enforced.
How to recognize it
The most striking feature is the shape of the filling: a star or an irregular shape with points and arms, surrounded by a thick rind of rhyolite. On the balls from the Teufelskanzel the rind is ocher to reddish brown, rough and often crossed by light fibers and dark cracks. The agate bands run parallel to the points, often violet, bluish gray and white at the edges, warm brown and honey tones inside. A special feature of this site: many pieces glow bright green under long-wave UV light, which is attributed to incorporated uranyl ions.
It is most easily confused with agates from basalt or melaphyre, such as those found along the Nahe. These, however, sit in roundish to almond-shaped cavities without points and have a thin, greenish to gray outer skin instead of a thick rhyolite rind. Geodes are hollow balls too, but their wall is usually evenly thick and their interior round rather than star-shaped.
Where it occurs
Rhyolite agates occur wherever silica-rich volcanic rocks with lithophysae have been preserved. World famous are the thundereggs from Oregon in the USA, where they have even been named the state rock, along with deposits in Mexico and Australia. In Germany the best known are in Saarland and Thuringia: the Teufelskanzel on the Leistberg near Oberthal in the Nohfelden rhyolite massif, and the Schneekopf balls from the Schneekopf in the Thuringian Forest.
The agates of the Teufelskanzel are mostly 6 to 14 centimeters in size. Besides the star-shaped fillings there are, very rarely, pieces with horizontal, plane-parallel banding, so-called layered agates. The rock pulpit itself is protected as a natural monument and can be reached via the Rötelsteinpfad trail.
Profile
| Mineralogy | |
| Classification | agate (banded chalcedony) as the filling of lithophysae in rhyolite; in the trade also called thunderegg |
| Locality in this example | Teufelskanzel, Leistberg near Oberthal, district of St. Wendel, Saarland; Nohfelden rhyolite massif, Rotliegend (Permian) |
| Chemical formula | agate: SiO₂; rhyolite: silica-rich volcanic rock of quartz and feldspar |
| Mineral class | oxides (agate) |
| Crystal system | trigonal, microcrystalline; visible quartz crystals in the remaining space |
| Mohs hardness | agate 6.5 to 7, rhyolite rind somewhat softer |
| Density | about 2.6 g/cm³ |
| Color | agate gray, brown, light to dark blue, with a violet tinge, rarely yellow or red; rind ocher to reddish brown |
| Streak | white |
| Luster | agate vitreous after polishing, rind dull to silky |
| Transparency | agate translucent in thin bands, rind opaque |
| Fracture | agate conchoidal, rind uneven |
| Cleavage | none |
| Special feature | many pieces from the Teufelskanzel glow green under long-wave UV light |
| Processing | |
| Difficulty | demanding: two materials of different hardness in one piece, plus cracks in the rind and open druses |
| Sawing | hard material; the position of the cut decides whether the star is hit in full or only one arm |
| Grinding & polishing | the agate takes a high gloss, the softer rind wears away faster and needs even guidance |
| Drilling | possible, chipping in the rind is possible; for thin slices a bail ring is the safe solution |
| Typical risks | cracks in the rind, loose quartz points in the remaining space, chipping rind edges |
| Suitable for | cut as a matching pair, flat polish, slices, pendant, cabochon from the agate area |
Processing notes
Rhyolite agate is processed at the rate for hard materials. What makes it special is that two different materials come together in one piece: the hard agate inside and the somewhat softer, often cracked rind of rhyolite. During grinding the rind wears away faster than the agate, so even guidance is important to keep the surface flat and stop the edges from chipping.
Here too, the most important step is the first cut. It decides whether the star is hit across its full extent or only one of its arms. If the ball is split down the middle, two mirror-image halves result, a matching pair. If a space with quartz crystals remains inside, it can be left open, although the points are delicate.
What I can do with your find:
- Sawing: splitting balls, matching pairs and slices
- Grinding and polishing: from a rough polish to a high gloss
- Drilling: for pendants, for thin slices alternatively with a bail ring
- Turning it into jewelry: from find to finished piece
You can work out the cost yourself with the stone processing calculator right away. Not sure whether your nodule is a rhyolite agate? Send me a photo, and I will take a look.
From the workshop
I have already sawn open and polished several rhyolite agates from the Teufelskanzel in my workshop. In the first piece below, the agate star fills almost the whole ball, with violet and bluish gray bands at the edge and honey brown agate inside, exactly the structure from stage 3. In the second piece the center has remained open, and quartz crystals sit in the remaining space.
Matching pieces in the shop
- Teufelskanzel Top Agate: large star, polished cut
- Rhyolite agate with quartz druse
- Rhyolite Agate from Germany
Frequently asked questions
What is a thunderegg?
That is the name for balls of rhyolite or similar volcanic rock whose interior is filled with agate, jasper, chalcedony or opal. The name comes from a legend of the Native Americans in Oregon, according to which thunder spirits threw the stones at each other in a quarrel. They have nothing to do with eggs.
Why is the agate star-shaped and not round?
Because it fills a cavity that cracked open in a star shape as the rhyolite ball cooled. Agates from basalt, on the other hand, fill round gas bubbles and therefore become almond-shaped.
Am I allowed to collect at the Teufelskanzel myself?
Digging there has been prohibited since the early 1980s, and the ban is enforced. The rock pulpit is also protected as a natural monument. If you hike there, please leave the slope and the rocks alone.
Why do some pieces glow under UV light?
Many agates from the Teufelskanzel contain traces of uranium in the form of uranyl ions. Under long-wave UV light they therefore glow bright green, especially in areas with coarser quartz.
Can I have a single nodule processed, or is there a minimum quantity?
Single pieces are the norm. Most orders are exactly that: a single nodule or slice someone brought along.
Sources
- Teufelskanzel, Leistberg, Oberthal, Saarland. In: agates.click, locality description 2021.
- Agates of the Teufelskanzel near Oberthal, Saarland, Germany. In: Gems & Gemology, Summer 2013 (after Lapis).
- Rhyolith-Steinbruch Oberthal. In: Naturpark Saar-Hunsrück, Geopunkte Permokarbon.
- Thundereggs, Lithophysen. In: steine-und-minerale.de, basics magazine section.
- Lithophysa. In: Wikipedia, The Free Encyclopedia.