Petrified wood is a fossil, a three-dimensional record of ancient plant life transformed into stone. This geological process, known as permineralization, occurs when the original organic material of a tree is replaced by dissolved minerals over millions of years. Groundwater rich in silica, often derived from volcanic ash, flows through the buried wood, depositing minerals like quartz, chalcedony, or opal into the cellular structure. The resulting specimen is an inorganic replica, a rock that perfectly preserves the form and texture of the original wood. Authenticating a piece requires examining specific visual features and testing its physical properties.
Visual Cues and Surface Texture
The most telling sign of petrified wood is the preservation of the original tree’s cellular structure, visible on the surface or in a cross-section. Features like growth rings, bark texture, and wood grain are often clearly visible, even though the material is now entirely mineralized. This level of detail is retained because the mineral replacement copies the intricate internal framework of the wood.
Another noticeable characteristic is the luster of the stone, often described as waxy or glassy, which is typical of silica minerals like chalcedony or quartz. This hard, smooth texture is distinctly different from the rough, fibrous feel of regular wood or the dull, earthy texture of common sedimentary rock. The vibrant coloration is also a clue; pure silica is colorless, but trace elements introduced during permineralization create a spectrum of hues. For example, iron oxides produce reds, browns, and yellows, while manganese results in pinks, purples, or blacks. Copper, cobalt, or chromium can introduce shades of green or blue, reflecting the mineral composition of the surrounding environment.
Testing the Hardness and Density
A method for identifying petrified wood is testing its hardness, which indicates its mineral composition. Since the organic material has been replaced primarily by silica, authentic petrified wood registers between 6.5 and 7 on the Mohs scale of mineral hardness. This is significantly harder than steel, which ranks around 4.5.
To perform a field test, attempt to scratch the specimen with a steel knife or a common nail. If it is true petrified wood, the steel tool will leave no mark or may be scratched itself. A more definitive test involves using a piece of glass, which has a Mohs hardness of about 5.5; a genuine specimen should easily scratch the glass surface.
When handling the specimen, you will notice its weight, a property directly related to its density. Mineral replacement makes petrified wood significantly denser than waterlogged or dried-out wood, causing it to feel “rock-heavy.” The specific gravity, which measures density relative to water, ranges from 2.62 to 2.66, reflecting the density of quartz.
If the specimen is broken, the fracture pattern provides a further physical confirmation. Quartz and chalcedony often display a conchoidal, or shell-like, fracture with smooth, curved surfaces and sharp edges. This is unlike the splintering break of organic wood and is a characteristic trait of the fossilized material.
Distinguishing Petrified Wood from Common Imitations
Differentiation requires applying visual and physical tests to rule out common look-alikes, both natural and artificial. Natural materials like chert or jasper can resemble petrified wood in color and hardness. However, they lack the key identifying feature: the distinct and preserved cellular structure, growth rings, and wood grain that are the hallmark of a true fossilized tree.
Wood stained with iron minerals or driftwood may feel heavy and appear rock-like. These specimens will fail the Mohs hardness test because the original organic cellulose is still present, allowing them to be scratched by steel. The retention of organic material means they will not exhibit the glassy luster or conchoidal fracture of fully silicified wood.
Artificial imitations, such as resin or plastic molded to look like wood, are common fakes. These fakes are much lighter than genuine petrified wood and will fail both the hardness and density tests, often feeling soft or warm to the touch. The grain patterns on these synthetic pieces may also appear too uniform or unnaturally blurry compared to the microscopic detail present in a naturally fossilized specimen.