How Petrified Wood Forms: When Trees Turn to Stone

Petrified wood is one of the most recognizable fossils. A polished slice can show tree rings, bark, and knots, yet it is heavy, cold, and hard enough to scratch glass. It is no longer wood at all. It is a stone copy of the original tree, created by the slow mineral process described in How Fossils Form.

Step One: Rapid Burial

Petrification starts when a fallen tree is buried quickly, before it can rot. River floods, mudflows, and volcanic eruptions are especially effective. Volcanic ash is important for another reason: it is rich in silica, the same material that makes up quartz. As water passes through buried ash, it dissolves silica and carries it into the buried logs.

Step Two: Minerals Fill the Cells

Wood is made of countless microscopic cells with hollow interiors and walls of cellulose and lignin. Silica-rich groundwater seeps into these spaces and deposits silica inside them, a process called permineralization. Over long periods, the original organic material may also break down and be replaced by silica, leaving a stone that preserves the structure in fine detail.

The silica often begins as opal, a noncrystalline form, and gradually converts to microcrystalline quartz, or chalcedony. That is why petrified wood is usually hardness 7 or close to it, like the quartz varieties covered in Quartz and Its Many Varieties.

Where the Colors Come From

Pure silica is colorless or white, so the colors in petrified wood come from other elements carried in the groundwater:

Iron oxides produce reds, oranges, yellows, and browns.
Manganese oxides can produce pinks and blacks.
Carbon left over from the original wood often produces black and gray.
Copper, chromium, and cobalt can produce rarer greens and blues.

Because mineral content shifted as groundwater moved, a single log can show several colors in bands or patches.

What Petrified Wood Preserves

Well-preserved petrified wood can show growth rings, which record seasonal growth and help scientists study ancient climates. Under a microscope, thin sections may show individual cells clearly enough to identify the type of tree. Some deposits preserve whole trunks still standing upright, buried where they grew. The study of this material is part of paleobotany, the branch of paleontology focused on ancient plants.

Famous Deposits

Petrified wood occurs worldwide, but a few sites are especially well known:

Petrified Forest National Park, Arizona, preserves huge, colorful logs from the Late Triassic Period, more than 200 million years ago.
Ginkgo Petrified Forest State Park, Washington, preserves wood buried by ancient lava flows.
Yellowstone National Park contains petrified trees buried upright by volcanic debris.

Removing petrified wood from national parks and many other protected lands is illegal. Material sold in rock shops typically comes from private land where collecting is allowed. The basics of where collecting is and is not permitted are covered in Starting a Rock Collection.

Petrified Wood Versus Similar Fossils

Not all fossil wood is petrified. Wood preserved as a carbon film or as coal has lost its three-dimensional structure. Jet, the black organic gem described in Organic Gems, also comes from wood but forms under different conditions and remains largely organic. Opalized wood, found in places such as Australia and Nevada, is wood replaced by opal that never converted to quartz, and it is softer and more fragile than typical petrified wood.