Petrified Wood

The single most telling clue is that petrified wood looks like wood but behaves like stone. Look closely at the surface and you should see the unmistakable signature of a tree: parallel grain running along the length of the piece, concentric growth rings on a cut end, and sometimes knots, bark texture or the radiating lines of wood rays. Yet when you pick it up, it is cold, dense and far heavier than any piece of real wood the same size, and it cannot be dented with a fingernail or cut with a knife. That contradiction — wood pattern, stone body — is what sets petrified wood apart at a glance.

Two quick tests confirm it. First, hardness: fully silicified wood is about 6.5–7 on the Mohs scale, so it will scratch glass and resist scratching by a steel blade, exactly like quartz. Second, look for the combination of features together — plain agate or jasper can show bands and swirls, but it will not show true, directional wood grain with growth rings. If you see a stone that has the color and glassy break of agate but is organized along the grain of a log, with rings and fibrous structure, you are almost certainly holding petrified wood rather than an ordinary banded chalcedony.

Petrified wood forms through a process called permineralization. When a tree dies and is quickly buried by sediment, volcanic ash or mud, it is sealed away from the oxygen and microbes that would normally rot it. Groundwater then seeps through that buried wood, and where the water is rich in dissolved silica — often because volcanic ash nearby is breaking down — the silica precipitates inside the wood. It fills the empty spaces and pores first, and over very long spans of time it also replaces the organic cell walls molecule by molecule. Crucially, this happens slowly and gently enough that the original cellular template is copied in mineral, which is why the grain, rings and even fine cell structure survive.

The silica usually crystallizes as chalcedony or quartz, the same family as agate, which is why so much petrified wood is hard, glassy and capable of taking a high polish. The vivid colors come along for the ride: iron oxides tint the stone red, orange, brown and yellow, manganese can produce black, blue and purple tones, and combinations of trace elements give the rainbow logs of the American Southwest their banded hues. Most of the famous petrified wood, including the Arizona deposits, dates from the Triassic Period and is on the order of 200 million-plus years old, though petrified wood of many different ages is found around the world.

Collectors and geologists distinguish petrified wood mainly by how it was preserved and how richly it is colored. The most prized material is "agatized" or "opalized" wood, in which the replacing silica is fine, translucent chalcedony or opal that polishes to a brilliant, gem-like finish — Arizona "rainbow" wood is the classic example. Plainer pieces, sometimes called simply silicified wood, are more opaque and earthy in tone but still show the grain clearly. Where the preserving mineral was something other than silica, you get rarer variants: wood permineralized with calcite, pyrite or other minerals exists, though silica-rich types are by far the most common in the trade.

Petrified wood is also categorized by the kind of tree it came from, because the underlying botanical structure is often identifiable. Many famous specimens are from ancient conifers (the Arizona logs are largely an extinct araucaria-type conifer), while others preserve palm, cycad or hardwood structure, each with its own internal pattern. For identification purposes, the practical varieties you will meet are: high-color polished rounds and slabs, rough limb casts and log sections, and small tumbled pieces — all united by the wood-grain-in-stone signature regardless of the species or the exact preserving mineral.

Most petrified wood is affordable and widely available, from inexpensive tumbled chips and small slabs up to large, dramatic polished tabletops and bookend pairs. Value rises with strong, varied color, sharp and well-preserved grain and rings, a high polish, larger size, and identifiable species or a documented locality. Vivid Arizona-style rainbow rounds and finely figured slabs command the most, while gray or brown pieces with muddy grain are the most modest. Note that collecting petrified wood inside protected areas such as Petrified Forest National Park is illegal; reputable material on the market comes from private land and commercial claims.

Outright fakes are uncommon, because real petrified wood is cheap enough that copying it rarely pays, but a few traps are worth knowing. Some "petrified wood" sold as novelties is actually dyed plain agate, glass, resin or even concrete cast and stained to look woody — tell-tale signs are color that sits on the surface rather than running through the stone, a suspiciously light or warm feel (glass and resin feel different from quartz), visible mold seams or air bubbles, and grain that looks painted on rather than carved into the structure. Genuine petrified wood is hard (it will scratch glass), heavy, and shows wood grain and growth rings that continue consistently through the body of the piece, not just on the surface. A more common issue than fakes is ordinary fresh wood being mistaken for petrified wood; the stone test settles it instantly, since real petrified wood is rock-hard and far too heavy to be wood.