Markgräfler Bohnerz jasper is a Jurassic chert colored yellow and red by iron. It formed around 155 million years ago as a siliceous nodule in the Jurassic sea, survived the dissolution of the surrounding limestone and today lies in the red clay of the Bohnerz formation between Müllheim, Schliengen and Kandern. With a Mohs hardness of 6.5 to 7 it is one of the hard materials. It is one of the characteristic minerals of the Markgräflerland.
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How Bohnerz jasper formed
Its story has two chapters separated by a hundred million years: first the siliceous nodule forms, and much later it gets its color.
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The nodule grows in the Jurassic sea
In the Late Jurassic, today's Markgräflerland lay beneath a warm, shallow sea. Fine lime mud collected on the bottom, layer upon layer over millions of years.
Siliceous sponges lived in this sea, with supporting skeletons made not of lime but of silicon dioxide. When they died, their needles sank into the mud and dissolved again in the pore water. The dissolved silica migrated through the soft ground until it precipitated again at certain spots and accumulated.
There, over very long periods, knobbly bodies of dense, microcrystalline quartz grew: chert. Because they formed in soft mud and not in a cavity, they are usually rounded and have no sharp edges. The tiny microfossils enclosed in the process still reveal today which layer a piece comes from.
The limestone dissolves
Around a hundred million years later the seabed had become land, and in the early Tertiary a humid tropical climate prevailed here.
On its way through the air and soil, rainwater absorbs carbon dioxide and becomes slightly acidic. That is exactly what dissolves limestone. Over millions of years this water ate its way into the limestone from above and created a karst landscape of fissures, funnels and pockets. At the surface, dolines formed, bowl-shaped hollows where the ground gave way.
The limestone disappeared. The chert nodules did not: quartz is practically immune to this kind of weathering. They stayed behind while everything around them sank away, and collected in the fissures and hollows.
Iron colors the stone
What the dissolved limestone left behind was a tough, deep red residual clay, the red clay. Its color comes from iron, which was present in the limestone only in traces and became more and more concentrated as it dissolved.
In this red clay, bean-shaped iron ore concretions formed, usually between nine and fifteen millimeters in size. They give Bohnerz, literally bean ore, its name, and until the 19th century iron was smelted from them in the Markgräflerland.
For the jasper this stage is the decisive one. For millions of years the chert nodules lay in this iron-saturated environment. Iron oxides and iron hydroxides penetrated their pore structure and soaked them from the outside in. The inconspicuous gray chert became the yellow and red flamed jasper. Where the iron did not reach, the white and gray variety remained. Both still lie side by side in the same clay today.
Geologists group these deposits as the Schliengen Formation; older literature calls them Siderolithic or the Bohnerz formation.
The plow brings it to light
Today the Bohnerz formation lies beneath the fields of the foothill zone, in many places only one plow depth below the surface. Every year the plow brings new nodules to the surface.
From the outside they look inconspicuous: a brown, earth-crusted pebble that gives no hint of what is inside. Only the cut shows the banding and the core. And because you can never be sure from the outside whether a nodule is silicified all the way through or whether lime remnants and hairline cracks are hidden inside, this only ever becomes clear when it is cut open.
How to recognize it
The most striking feature is the play of colors: the yellow and red pieces are so distinctively flamed that they can usually be told apart well from other Bohnerz cherts. In addition there is the conchoidal fracture with sharp edges, exactly the property that made it sought after as a tool stone in the Stone Age.
It is most easily confused with other cherts from southern Baden: the Muschelkalk chert from the Schönberg near Freiburg, the Tertiary freshwater chert from the Tüllinger Berg near Lörrach, or the Middle Jurassic tabular chert from the Emmendingen foothill zone. If you are unsure, the enclosed microfossils are the most reliable criterion, although they are not present in every piece.
Where it occurs
In the Markgräflerland, the actual primary deposits of the jasper are limited to the Isteiner Klotz near Lörrach. What is found today, on the other hand, almost always comes from secondary deposits, that is, from the redeposited Bohnerz formation. Its distribution runs through the foothill zone between Müllheim in the north and Efringen-Kirchen in the south, with its focus in the triangle of Müllheim, Schliengen and Kandern.
Auggen lies right in the middle of this area. At the Steinacker site near Auggen there is an occurrence of yellow and red Bohnerz jasper exposed at the surface by erosion. This site is considered one of the oldest raw material extraction sites in Germany.
At least 27,000 years of use
Stone Age
The finds from the Steinacker in Auggen can be dated to the Gravettian, the middle Upper Paleolithic about 27,000 to 15,000 years ago. Tanged points were knapped from this material there; the largest surviving example measures around eight centimeters. In the Magdalenian, Markgräfler Bohnerz jasper traveled far with the hunter groups. Individual pieces have been found up to 300 kilometers to the north at the Middle Rhine site of Gönnersdorf. In the Neolithic, mining was added: at Kleinkems on the Isteiner Klotz, people dug for jasper between about 5400 and 2600 BC.
Renaissance and Baroque
From around 1600 an extensive trade began, and in the 17th century its processing reached its heyday. Between roughly 1400 and 1800, Freiburg and neighboring Waldkirch were among the most important stone cutting centers in Europe. Beads, knife handles, cane knobs and ornamental pieces were made there from Markgräfler Bohnerz jasper, practically as mass-produced goods. Excavations in Freiburg's old town have brought the waste from these workshops to light by the thousands.
The stone also made it to the European courts. In 1601 Emperor Rudolf II summoned two Freiburg master stone cutters together with six apprentices to his court in Prague. In the Miseroni workshops there, Bohnerz jasper was worked into ornate vessels: an oval bowl of banded Bohnerz jasper from the first third of the 17th century is now in the Kunstkammer of the Kunsthistorisches Museum in Vienna, as is a flower vase from 1678 whose tulips are cut from Bohnerz jasper, chalcedony, agate and amethyst. A tabletop made in Prague between 1619 and 1623 for Prince Karl I of Liechtenstein consists of 80 to 85 percent colored Bohnerz jasper.
It was known in Florence too: in the collections of the Opificio delle Pietre Dure, the white-gray and yellow-red Bohnerz jaspers are listed as “Diaspro d’Alsazia”. The flamed pieces were particularly popular for depicting tulips and other flowers in Florentine pietra dura work. And the Margraves of Baden had seal stones, snuff boxes and ornate vessels made from the stone of their own uplands, which went as diplomatic gifts to European courts.
Until 1950, then silence
The August Wintermantel gem cutting works in Waldkirch, the last grinding mill in the Breisgau, still regularly worked Markgräfler Bohnerz jasper on its large sandstone wheels until around 1950. After that the material fell silent.
It never disappeared completely, though. In Hach, a district of Auggen, Fritz Schmidlin ran a cutting workshop and built up a collection over decades. He also devoted three volumes to the stone, describing its formation, localities and the Stone Age tools from Auggen. Without this work we would know much less about Bohnerz jasper.
Profile
| Mineralogy | |
| Classification | chert (silex), Rauracian chert of the Lower Malm, referred to as jasper |
| Chemical formula | SiO₂, microcrystalline, with Fe₂O₃ and FeO(OH) as color carriers |
| Mineral class | oxides |
| Crystal system | trigonal, microcrystalline to cryptocrystalline |
| Mohs hardness | 6.5 – 7 |
| Density | 2.58 – 2.64 g/cm³ |
| Color | yellow and red flamed to deep red |
| Streak | white to pale reddish |
| Luster | dull on fracture, vitreous to greasy after polishing |
| Transparency | opaque, translucent at the edges of thin slices. Rarely a slight transparency in thinner slices |
| Fracture | conchoidal, hence its suitability as a tool stone |
| Cleavage | none |
| Processing | |
| Difficulty | demanding: hard and tough, but sensitive at cracks and fissures. Things often turn out differently than planned, so you need a certain flexibility. |
| Sawing | hard material, slow feed, very demanding; care is needed, check carefully whether the material has hidden flaws. |
| Grinding & polishing | takes an excellent polish; the color flaming only shows fully at a high gloss |
| Drilling | drills well to moderately. I recommend a high speed with low feed force. Depending on the iron content of the jasper you have to be very careful: the water must come out of the drill under pressure to carry the iron sludge away properly, otherwise the drill jams and breaks. |
| Typical risks | hairline cracks from frost weathering, lime inclusions, unsilicified zones inside the nodule, which can be a risk with automatic saws depending on how the feed works |
| Suitable for | flat polish, cabochon, pendant, jewelry set, key ring |
Found some Bohnerz jasper yourself? I saw, grind and polish it in my workshop in Auggen, as a slice, a flat polished piece or a finished piece of jewelry. The stone processing calculator shows you the price straight away, and the inquiry is non-binding.
Processing notes
Bohnerz jasper is a hard material. At 6.5 to 7 on the Mohs scale, it is on the same level as agate and flint.
A nodule is first cut open so that you can see what is inside. From the outside you cannot tell whether a find is silicified all the way through or whether lime remnants, unsilicified zones or hairline cracks from frost weathering are hidden inside (usually you cannot see it). Only the cut reveals the flaming, and whether further work is worthwhile.
Grinding and polishing then show the real strength of the material: it takes an excellent polish, and the color flaming only shows fully at a high gloss. What looks like a brown pebble in the field can be banded deep red and golden yellow as a flat polish.
What I can do with your find:
- Sawing: cutting nodules open, slices and flat polish blanks
- Grinding and polishing: from a rough polish to a high gloss
- Drilling: for pendants, alternatively with a bail ring
- Turning it into jewelry: from found stone to finished piece
You can work out the cost yourself with the stone processing calculator right away. If you are not sure whether your find is Bohnerz jasper at all: Send me a photo, and I will take a look.
From the workshop
What the material looks like once it has been fully worked is best seen in the pieces themselves:
- Bohnerz Jasper Gallery: pieces that are not for sale
- Flat polished pieces: the shape in which the flaming shows best
- Pendants and jewelry sets
Frequently asked questions
Is Bohnerz jasper the same as Bohnerz?
No. Bohnerz are the bean-shaped iron ore concretions from which iron used to be smelted. Bohnerz jasper is the siliceous stone that lies together with them in the same red clay and was colored by their iron. So the name comes from where it is found, not from its composition.
Where am I allowed to look for Bohnerz jasper?
Most is found on harvested fields after plowing. But fields have owners, so ask first. Collecting is prohibited in nature reserves and at protected geotopes.
How can I tell whether a piece is worth it?
Clues are a fresh break, the weight, experience or a worn spot: if it shows a dense, glassy looking texture, the piece is well silicified. If the broken surface looks chalky or grainy, it is more likely unsilicified limestone.
Can I have single finds processed, or is there a minimum quantity?
Single pieces are the norm. Most orders are exactly that: a single stone that someone found themselves.
Sources
- Michael J. Kaiser: Der Markgräfler Jaspis – Einblicke zur (unterschätzten) Kulturgeschichte eines Feuersteins. In: Man and Mining – Mensch und Bergbau. Der Anschnitt, Beiheft 16, Bochum 2003, pp. 215–225.
- Michael J. Kaiser: Bohnerz und Bohnerzgewinnung im Markgräfler Hügelland. In: W. Konold, R. J. Regnath, W. Werner (eds.): Bohnerze. Jan Thorbecke Verlag, Ostfildern 2019.
- Fritz Schmidlin: Bohnerzjaspis. Entstehung – Faszination. Volumes I to III, J. H. Röll Verlag, Dettelbach 2004–2009.
- Otto Wittmann: Markgräfler Bohnerz-Jaspis in der Florentiner Pietra-dura-Arbeit. Das Markgräflerland 6 (37), 1975, pp. 321–322.
- Landesamt für Geologie, Rohstoffe und Bergbau Baden-Württemberg: Tertiäre Residuallehme und Bohnerzformation.