Petrified wood is a three-dimensional fossil that looks like a piece of wood but possesses the hardness and density of stone. It forms when ancient wood is buried quickly and the organic material is slowly replaced by minerals, primarily silica in the form of quartz, agate, or opal. This process, called permineralization, results in a perfect stone replica of the original tree, often preserving details down to the cellular level. The resulting material is a unique blend of wood’s familiar form and a rock’s striking color and texture.
The Preservation of Wood Structure
The most striking visual aspect of petrified wood is its faithful retention of the original tree’s structure. This fidelity is achieved because mineral replacement happens at a microscopic level, copying the layout of the wood’s cell walls. The internal anatomy is often preserved so accurately that scientists can sometimes identify the original tree species under a microscope.
A cross-section of a petrified log reveals the familiar pattern of annual growth rings, appearing exactly as they would in a freshly cut tree. These rings show the tree’s history, with variations in color and texture marking seasonal boundaries. Features like knots, worm tracks, and the texture of the original bark are frequently preserved on the exterior. The wood structure is maintained because minerals fill the tiny void spaces and then replace the decaying cellulose and lignin, using the cell structure as a template.
The Role of Minerals in Coloration
The vibrant and diverse colors seen in petrified wood come from trace elements present in the mineral-rich water during petrification, not the original tree. As silica-based minerals like quartz and chalcedony replace the wood, impurities, often metals, are incorporated into the stone structure. These impurities act as natural pigments, creating a wide spectrum of hues.
Iron is the most significant colorant, producing a range of warm tones depending on its oxidation state. Iron oxides like hematite typically create reds, pinks, and deep browns, while hydrated iron oxides like goethite result in yellows and oranges. Manganese is responsible for less common colors, yielding pinks, purples, and sometimes dark grays or black.
Striking blues and greens are rare, indicating the presence of trace amounts of copper, cobalt, or chromium. When no trace elements are present, pure silica replacement results in a translucent, white, or clear appearance. Specimens often display multiple colors in complex, banded patterns due to changes in the mineral-laden water’s chemistry over time.
Physical Characteristics and Texture
While it looks like wood, petrified wood is physically a rock, possessing significantly higher density and hardness than its organic predecessor. Its weight is often surprising, as the light organic material has been replaced by heavy mineral matter. The hardness of petrified wood, usually 6.5 to 7 on the Mohs scale, is comparable to granite or quartz.
The surface texture and luster vary depending on weathering and whether the specimen has been polished. Raw, unpolished pieces often have a dull or waxy luster and a slightly rough, splintery fracture where the mineralized wood grain has broken. When polished, the surface becomes smooth and highly vitreous due to the quartz-based composition. This high hardness makes the material resistant to scratching and wear, contrasting sharply with the soft nature of the original wood.