Short answer: petrified wood is a fossil: wood buried quickly enough to escape decay, then soaked by mineral-rich groundwater until the plant tissue was filled and replaced, usually by silica. Most of it is quartz or chalcedony (agatized wood, Mohs about 7); some is opal (opalized wood, softer and lighter). In the United States you may not take any from a national park, while BLM land allows 25 pounds plus one piece a day for personal use. Legal stock for sale comes from private land.
What is petrified wood made of?
Mostly silica. Geology.com defines petrified wood as a fossil that forms when plant material is buried by sediment and protected from decay, after which groundwater rich in dissolved solids replaces the original material with silica, calcite, pyrite or another inorganic material such as opal. The National Park Service at Petrified Forest puts it more vividly: the wood there is made up of almost solid quartz, and each piece is like a giant crystal.
That means "petrified wood" covers a few different stones that share a shape:
| Type | Main mineral | Mohs hardness | Specific gravity | What it looks like |
|---|---|---|---|---|
| Agatized (chalcedony) wood | Microcrystalline quartz | 6.5 to 7 | 2.59 to 2.61 | Waxy to glassy polish, bark and rings in color |
| Opalized wood | Opal | 5.5 to 6 | 2.04 to 2.23 | Lighter, often creamy or brown, may craze |
| Carbonate or pyritized wood | Calcite or pyrite | 3 (calcite) | Varies | Soft, fizzes in acid, or brassy and prone to decay |
The hardness, gravity and refractive index figures for opal versus chalcedony come from Geology.com, which adds that both can safely be called "silicified wood" when you are not sure which you have, since telling them apart takes testing. The spot refractive index of opal (1.39 to 1.48) is clearly below chalcedony's 1.53 to 1.54, which is how a gemologist settles it.
How does wood turn to stone?
There are two overlapping processes, and most specimens show both.
Permineralization fills the empty spaces. Wood is porous: the cell cavities and vessels that carried water in the living tree are open tubes. Silica-rich water seeps in and deposits mineral inside them, locking the cell walls in place.
Replacement goes further. As the original cellulose and lignin slowly break down, mineral takes their place molecule by molecule, which is why a polished slice can show growth rings, rays and even insect borings in stone.
The silica usually comes from volcanic ash. At Petrified Forest, the NPS explains that logs washed into an ancient river system over 200 million years ago and were buried quickly and deeply enough that oxygen was cut off and decay slowed. Silica dissolved from volcanic ash then absorbed into the porous wood over hundreds and thousands of years and crystallized within the cellular structure, replacing the organic material. The park's FAQ adds that the ash blew in from 200-million-year-old volcanoes in what is now California, southern Nevada and western Arizona during the Late Triassic (NPS Petrified Forest FAQ). The colors come from impurities in the quartz: the NPS names iron, carbon and manganese.
Two details explain features buyers notice. Cracks in the logs sometimes filled with larger crystals, so the park records clear quartz, purple amethyst, yellow citrine and smoky quartz inside some logs. And the famous "sawn" look is natural: the NPS says that during the uplift of the Colorado Plateau, starting about 60 million years ago, the buried logs were under so much stress that they broke like glass rods, leaving clean, evenly spaced fractures.
Where does petrified wood come from?
Geology.com stresses that petrified wood is not rare. It occurs wherever volcanic ash, mudflows or sediments buried wood and silica-bearing water followed. In Arizona's Chinle Formation, about 225 million years ago, conifers up to 9 feet in diameter and 200 feet tall grew on a tropical lowland, according to the same article; most of the park's trees carry the scientific name Araucarioxylon arizonicum.
Other notable American localities Geology.com lists include the petrified palm of the Catahoula Formation in Louisiana, Texas and Mississippi, the Ginkgo Petrified Forest near Wanapum Reservoir in Washington, the Petrified Forest near Calistoga, California, and Escalante Petrified Forest State Park in Utah. Abroad, Western Australia produces "peanut wood", whose pale ovoid markings are borings made by a shipworm, a tiny clam. Indonesia is a major modern source of polished spheres, slabs and furniture; see the planned Indonesia origins guide.
Is it legal to collect petrified wood in the US?
It depends entirely on who owns the land.
National parks: never
National Park Service regulations at 36 CFR 2.1 prohibit possessing, removing, digging or disturbing nonfossilized and fossilized paleontological specimens and mineral resources in park areas. Geology.com confirms that collecting petrified wood at Petrified Forest is prohibited. The park's FAQ explains why: with more than half a million visitors a year, the park could be picked clean within a few years if collecting were allowed, and it protects only just over 20 percent of the original deposit.
Fossils on federal land are also covered by the Paleontological Resources Preservation Act, which the NPS fossil protection page says was enacted on March 30, 2009, within the Omnibus Public Land Management Act.
BLM land: limited personal collecting
Bureau of Land Management land is the main legal hunting ground, under a rule written specifically for petrified wood. The regulation at 43 CFR subpart 3622 sets the limits:
- 25 pounds plus one piece per person per day, without charge.
- 250 pounds per person per calendar year, with no pooling of quotas to get pieces larger than 250 pounds.
- Personal use only. Wood collected this way shall not be sold or bartered to commercial dealers.
- No explosives or power equipment such as bulldozers or tractors.
- Museum pieces over 250 pounds need a permit, issued only if the applicant certifies the specimen will be publicly displayed.
The BLM's rockhounding page adds that collection is generally not allowed on developed recreation sites and some other areas, and directs collectors to the local field office. Rules differ again on state land and national forests, so check before you dig.
Private land: with permission
The NPS FAQ is clear on where rock shops get their wood: every piece sold in gift shops comes from private lands outside the park, whose owners have the right to collect, mine and sell it. Geology.com warns that people have gone to jail for collecting petrified wood where removing it is a criminal act.
How do you judge quality?
Geology.com notes that only a small fraction of petrified wood is suitable for lapidary work: pieces with voids or closely spaced fractures do not polish well or break while being worked. The qualities that raise price:
- Completeness of silicification. Fully filled pores take a glassy polish; partly filled wood shows dull, pitted patches.
- Visible wood structure. Bark, growth rings and rays in sharp detail are what collectors pay for.
- Color. Reds, yellows and purples from iron and manganese command more than plain brown and gray.
- Cracks. Natural fractures are common; open cracks in a sphere or slab are a weakness, and filled cracks should be disclosed.
- Locality. Documented localities, especially closed ones, carry a premium for collectors.
How do you care for petrified wood?
Agatized wood is tough and needs only a damp cloth. Opalized wood is softer and can dry out and craze, so keep it out of strong heat. Carbonate wood scratches easily and etches in acids. Pyritized wood can decay in humid storage. Large rounds and tabletops are heavy and can split along old fractures if dropped, so lift them from the base.
What is petrified wood associated with in crystal traditions?
In crystal traditions, petrified wood is associated with patience, grounding and a sense of deep time, because it preserves a tree across millions of years. That is an association, not a property of the stone.
Choosing a petrified wood piece
Ask three questions: what mineral it is (agatized, opalized or other), where it came from, and whether the wood structure is visible. For spheres and polished pieces, look at the listing photos for grain running cleanly through the stone and for open cracks at the surface. Heavier for its size is a good sign, since a fully silicified piece is dense.
- Petrified Wood: small natural pieces at an entry price, ideal for seeing bark texture and growth rings close up.
- Petrified Wood Sphere: a polished sphere from Indonesia, listed as about 20 million years old; spheres cut across the grain show the rings as bands around the ball.
- 27.3 lb fossil wood sculpture: a large statement piece where the wood grain and quartz crystallization are both visible, for a display that needs scale.
Digital Towns Market
Petrified wood in stock
Frequently asked questions
Is petrified wood a crystal or a fossil?
It is a fossil made of minerals. The original wood was replaced and filled mostly by silica, so most petrified wood is quartz, chalcedony or opal that preserves the tree's structure. It is sold with crystals because it polishes and behaves like agate.
What is the difference between agatized and opalized wood?
Agatized wood is replaced by chalcedony, microcrystalline quartz, with hardness 6.5 to 7 and specific gravity about 2.6. Opalized wood is replaced by opal, which is softer at 5.5 to 6 and lighter at about 2.04 to 2.23.
Can I take petrified wood from Petrified Forest National Park?
No. Removing fossils or minerals from a national park is prohibited under 36 CFR 2.1. Petrified wood sold in nearby shops comes from private land outside the park, where owners may legally collect and sell it.
How much petrified wood can you collect on BLM land?
Up to 25 pounds plus one piece per person per day and 250 pounds per year, for personal use only, without power equipment. It may not be sold or bartered to commercial dealers. Pieces over 250 pounds need a museum-display permit.
How old is petrified wood?
It varies by locality. The logs at Petrified Forest in Arizona date to the Late Triassic, over 200 million years old. Much decorative Indonesian wood is far younger, with listings commonly giving ages in the tens of millions of years.
Sources
- Geology.com, What is Petrified Wood? How Does it Form?: https://geology.com/stories/13/petrified-wood/
- National Park Service, Petrified Forest National Park, Petrified Wood: https://www.nps.gov/pefo/learn/nature/petrified-wood.htm
- National Park Service, Petrified Forest National Park, Frequently Asked Questions: https://www.nps.gov/pefo/faqs.htm
- eCFR, 43 CFR Part 3620 Subpart 3622, Free Use of Petrified Wood: https://www.ecfr.gov/current/title-43/subtitle-B/chapter-II/subchapter-C/part-3620/subpart-3622
- eCFR, 36 CFR 2.1, Preservation of natural, cultural and archeological resources: https://www.ecfr.gov/current/title-36/chapter-I/part-2/section-2.1
- National Park Service, Fossil Protection: https://www.nps.gov/subjects/fossils/fossil-protection.htm
- Bureau of Land Management, Rockhounding: https://www.blm.gov/programs/recreation/rockhounding