Petrified Wood

Petrified wood, also called wood stone, is fossil wood whose cell structure was replaced by silica over millions of years while the organic material decomposed. Mineralogically it is chalcedony and opal, closely related to agate and jasper, usually with a Mohs hardness of 6.5 to 7. What makes it special: a polished section often still shows the growth rings, the grain and sometimes even the bark of the tree it once was.

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How petrified wood forms

From a standing tree to a stone that can be polished takes several steps spread over millions of years, and they only come together under very specific conditions.

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The volcano buries the forest

A volcanic eruption, of the kind that happened several times during the Permian in what is now Saxony and the Palatinate, buried parts of the forest of that time under hot ash and pumice lapilli. The sudden exclusion of oxygen was decisive: without air, the usual rot bacteria and fungi could not do their work, which would otherwise have decomposed a fallen trunk within a few years.

Not every buried tree turns to stone. It also takes water containing silica seeping through the sediment, and here it was the ash itself that supplied it: volcanic glass dissolves comparatively easily in damp ground and releases its silica into the groundwater.

Silica soaks the cells

While further layers of sediment cover the trunk, mineral-rich water seeps through the rock and penetrates the cell cavities of the wood. There, protected from pressure and currents, the dissolved silica has time to settle.

This happens molecule by molecule: chalcedony and opal are deposited on the inner walls of the wood cells, at first only as a thin coating, while the organic cell wall itself is gradually broken down. Because the mineral is deposited exactly along the existing cell structure, its shape is preserved, even though the substance has long since changed.

The growth rings turn to stone

This process continues over further millions of years and does not run at the same speed anywhere in the trunk. Some areas silicify completely and densely, others remain more permeable and only become fully saturated much later, if at all.

In the end nothing is left of the original organic substance, but its architecture is preserved down to microscopic detail: growth rings, the fine vessels of the wood, occasionally even bark or insect borings. This is exactly what sets petrified wood apart from any other chalcedony: it is a fossil, not a pure mineral.

Erosion exposes the trunk

Where streams and rivers have cut into the sediments, or where erosion simply removes layer after layer, petrified trunks come back to the surface. Their cross section then shows what millions of years underground have made of them.

At scientifically important sites such as the Petrified Forest of Chemnitz, collecting is prohibited for good reason, and the natural monument is reserved for research.

How to recognize it

The most reliable feature is the grain itself: growth rings running across the cut surface as concentric or parallel lines, sometimes knots or a bark margin. Genuine petrified wood shows this structure as completely regular and organic even on close inspection, not randomly mottled.

It is most easily confused with ordinary banded jasper, which lacks the wood structure, or with petrified bone, which shows a fine network of Haversian canals instead of growth rings. If you are unsure, a magnifying glass is usually enough to tell rings apart from random mottling.

Where it occurs

Best known worldwide are the huge deposits in Petrified Forest National Park in Arizona. In Germany the Petrified Forest of Chemnitz is by far the most important site: around 291 million years ago, in the Early Permian, an eruption of the Zeisigwald caldera buried a forest of tree ferns, horsetails and early conifers there. The finds are protected as a natural monument and exhibited in the Museum für Naturkunde Chemnitz.

Closer to the workshop lies a smaller but geologically related area: the North Palatinate Uplands and the Palatinate Forest, with sites near Imsbach, Wolfstein and Winnweiler, for example. This wood also comes from the Rotliegend and thus from the same volcanically shaped Permian basin that also produced the agates common around Idar-Oberstein on the Nahe. Quite independently of this, silicified wood of an entirely different origin turns up now and then: as loose pebbles in northern German gravel pits, mostly of Eocene age and carried in from the Baltic region by the Ice Ages.

Profile

A piece of petrified wood at home? 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, petrified wood lies in the same hardness range as agate and Bohnerz jasper. The real challenge is not the hardness but its irregularity: within one and the same piece, densely silicified, glass-hard zones alternate with softer, partly porous areas.

When sawing, the chosen cutting plane decides the result. A cross section through the original trunk shows the growth rings as concentric patterns, a lengthwise cut shows the grain as parallel stripes. Only a test cut shows which plane gives the most for a particular piece.

What I can do with your find or rough stone:

• Sawing: trunk cross sections, lengthwise cuts 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 rough 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 piece is petrified wood at all: Send me a photo, and I will take a look.

Matching pieces in the shop

• Petrified wood: an overview of all finished pieces
• Flat polished pieces: the shape in which growth rings and grain show best
• Pendants and jewelry sets

Frequently asked questions

Is petrified wood still real wood?

No. It has turned completely into mineral, chalcedony and opal, chemically pure silicon dioxide. Only the structure is preserved, not the substance. That is why it can be sawn, ground and polished like agate or jasper, and not like wood.

How can I tell whether a stone really is petrified wood?

Most reliably by the grain itself: concentric growth rings in cross section or parallel stripes in a lengthwise cut, sometimes with a bark margin. A magnifying glass helps to tell this apart from random mottling, which other jasper can show as well.

Is all petrified wood equally hard?

No, and when processing it, that is the real difference from agate. Depending on how complete the silicification was, glass-hard and softer, more porous zones alternate in one and the same piece.

Can I have single finds or rough stones processed, or is there a minimum quantity?

Single pieces are the norm. Most orders are exactly that: a single piece that someone brings along or sends in.

Sources

• Versteinerter Wald (Chemnitz). In: Wikipedia, Die freie Enzyklopädie.
• Versteinertes Holz. In: steine-und-minerale.de, fossil encyclopedia.
• Verkieseltes / silifiziertes Holz. In: Mineralienatlas Lexikon.
• Versteinertes Holz in der Pfalz. In: Mineralienfreunde der Pfalz.