Agate

Agate is the banded variety of chalcedony, and hardly any other mineral shows such a variety of forms: every nodule carries its own pattern, grown over millions of years in a cavity in volcanic rock. With a Mohs hardness of 6.5 to 7 it belongs, like Bohnerz jasper, to the hard materials. In my workshop it is above all one thing: the mineral most people bring along, whether as a find from their last holiday, an inherited rough nodule or a slice from a rock shop that is finally meant to become jewelry.

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How agate formed

Agate is not a sedimentary stone but a child of volcanism. Its story begins in liquid lava and ends, many millions of years later, as a round pebble, for example in a stream bed.

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The lava solidifies with gas bubbles

When basaltic or andesitic magma reaches the surface, the gas dissolved in it escapes as countless tiny bubbles. Most of them rise and escape into the air. But the lava cools from the outside in, and the crust that has already solidified traps some of the rising bubbles just before they can escape.

What remains is a volcanic rock full of small and large cavities, known as amygdales. Geologists then speak of amygdaloidal rock, known in the Black Forest by the old name melaphyre. At first each of these bubbles is just an empty space in the stone, round to oval in shape, exactly the shape many agates will have later.

The cavity fills up

The volcanic rock solidifies completely and from then on lies underground as solid rock. Over time, solutions containing silica penetrate the cavities through cracks and fine capillaries, released from the surrounding rock itself or from neighboring layers.

This solution is very dilute. It therefore takes a very long time before the first material settles at the bottom of the amygdale. This long period, described in scientific papers as diffusion processes at temperatures up to about 200 degrees, is exactly why agates show such an even, fine texture: there is simply no time for fast growth.

Layer by layer towards the center

Starting from the wall of the amygdale, layer after layer now grows inwards. Each layer forms from the same silica, but the composition of the solution changes slightly over time: sometimes it contains more iron, sometimes less, sometimes the solution is purer. This changing chemistry is recorded as a change of color, band by band, and in the end produces the characteristic concentric pattern.

If the cavity is filled completely in this way, a solid agate nodule forms. If the silica does not reach the center, a space remains in which individual crystals grow freely inwards instead. Then it is called a druse or geode rather than a solid agate, the same growth principle, just stopped one step earlier.

When the amygdaloidal rock weathers, the agate remains

The volcanic rock around the finished agate nodule is far less resistant than the dense, solid silica filling itself. Over millions of years wind, water and frost gnaw at the amygdaloidal rock and remove it grain by grain. The agate survives this process almost unchanged.

This is how agate nodules end up in stream and river beds, where they lie among ordinary pebbles and look no different from them, except for what is revealed when they are cut open. That is exactly what makes searching for agates in the field so appealing: from the outside you cannot tell anything from the gray, earth-crusted pebble.

How to recognize it and what it is confused with

The most reliable feature is the banding itself: concentric or straight stripes of alternating translucent and denser layers, which only become fully visible in a polished section or against the light. On a rough piece, the round, almond-shaped outline often already gives away its volcanic origin.

It is mainly confused with two close relatives. Jasper consists of the same microcrystalline quartz but is colored opaque by finely dispersed iron oxides and shows no translucent banding, exactly the difference that also separates Bohnerz jasper from an agate. Chalcedony, in turn, is mineralogically identical to agate but stays even and without the characteristic bands, usually in a single milky bluish gray. A particularly finely patterned variety, Crazy Lace agate, shows a tangled, lace-like pattern instead of concentric bands and is therefore traded under its own name.

Where it occurs, with a focus on Germany

Internationally, the most important deposits are in Brazil and Uruguay, along with sites in the USA, India and large parts of Africa. Germany's best known agate region lies along the Nahe, around Idar-Oberstein. There the stones formed around 290 to 285 million years ago in the Rotliegend, when intense volcanism piled up andesite and melaphyre and filled their gas cavities with silica. At first the agates could practically be picked up from the fields, later they were quarried from the host rock. From the late Middle Ages a cutting industry grew up along the local streams, and an organized trade is documented as early as 1454.

At some point the local deposits were no longer enough for this industry. In 1834 the first shipment of Brazilian agates arrived in Idar-Oberstein, sent by emigrated compatriots who had settled in Rio Grande do Sul, a region rich in agate. Since then the region has mainly processed imported material and remains a center of gemstone cutting to this day, even though some of the bright colors sold there, blue, pink and violet, come from artificial dyeing and not from nature.

Closer to my workshop lies the second German focus: the Black Forest. On the Gaisberg and near Baden-Baden you can find melaphyre amygdaloidal rock from the same volcanic period as along the Nahe. In the southern Black Forest, for example near Alpfen, a carnelian horizon in the Buntsandstein yields blood-red, iron-stained pieces that formed in the desert-like climate of the Triassic. And in the Münstertal valley, barely half an hour's drive from my workshop in Auggen, agates even occur in hydrothermal ore veins, colored bluish gray by finely dispersed galena. In addition, as everywhere on the Upper Rhine, there are agate pebbles rounded by the river in the Rhine gravel. Agate has no deposit of its own right on the doorstep here, as Bohnerz jasper does, but the region is anything but empty.

Profile

Found an agate, inherited one or brought one back from holiday? 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

At 6.5 to 7 on the Mohs scale, agate is just as hard as Bohnerz jasper and is charged accordingly at the rate for hard silicates.

Here too, the first cut is the most important step: it shows whether there is continuous banding inside, whether a cavity with crystals is waiting or whether fine stress cracks will make the stone break open during sawing. After that it is decided whether the rough piece becomes a flat polish with a visible pattern, a slice with a window polish that lets light through the thinnest bands, or a cabochon for a pendant and jewelry set.

What I can do with your find or rough piece:

• Sawing: cutting nodules open, slices and flat polish blanks
• 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 found stone to finished piece

You can work out the cost yourself with the stone processing calculator right away. Not sure whether your piece is an agate at all, or rather jasper or an ordinary pebble? Send me a photo, and I will take a look.

From the workshop

No other mineral that passes through my workshop shows such a range: every nodule brings its own pattern, and only the cut decides whether it becomes a calm gray band, a window polish with warm light shining through or a slice with an exposed crystal druse. I work the pieces in their natural color, without artificial dyeing, just as they come out of the stone.

Matching pieces in the shop

You will find finished agate pieces in several collections, depending on shape and cut:

• All agates: the whole range at a glance
• Flat polished pieces: the shape in which the banding shows best
• Pendants and jewelry sets
• Crazy Lace agate: the variety with the lace-like pattern

Frequently asked questions

Is every agate hollow inside?

No. If the banding grows all the way to the center, a solid nodule without a cavity forms. If growth stops earlier, a space remains in which individual crystals form, and then it is called a druse or geode.

What is the difference between agate and jasper?

Both consist of the same microcrystalline quartz. Jasper is colored opaque throughout by iron oxides and shows no translucent banding. Agate is the banded variety of the same mineral, translucent in thin layers.

Do you dye agates, as is common in Idar-Oberstein?

No. The strong blue, pink or violet shades you know from the trade almost always come from artificial dyeing. In my workshop every piece only gets the color it naturally brings along.

Where am I allowed to look for agates myself in the Black Forest?

At well-known sites such as the Gaisberg or around old mine dumps you can find something with a little patience, but in the field the pieces are often inconspicuous and hard to recognize. Important: ask first on private land, and collecting is prohibited in nature reserves and at protected outcrops.

Can I have a single find 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

• Achate von Idar-Oberstein. In: steine-und-minerale.de, regional magazine section.
• Einfuhr von Edelsteinen aus Brasilien. In: regionalgeschichte.net, regional history information system of Rhineland-Palatinate, Hunsrück / Idar-Oberstein.
• Hansjörg Becherer: Achate im Schwarzwald. Lecture summary, Geologischer Zirkel Biberach, May 2021.
• Achat. In: steine-und-minerale.de, mineral encyclopedia.