Petrified vs. agatized vs. opalized

Petrified wood — an umbrella term for wood turned to stone by mineralization. If the filling/replacement is mostly chalcedony/agate (quartz), it’s often called “agatized wood.” If silica crystallized as opal (hydrated silica), it’s “opalized wood.” Many specimens show a mix of these phases.

What sets it apart

Unlike coal or lignite (altered, carbon-enriched remains), petrified wood preserves structure. Under a magnifier you can distinguish tree groups — conifers or hardwoods — by the “baked” anatomy in the stone.

1) Rapid burial and isolation

Logs are covered by volcanic ash, river sediments, or landslides. Oxygen decreases, decay slows, and wood microarchitecture remains intact enough for minerals to enter.

2) Silicon in solution

Groundwater, often circulating through ash or silica-rich rocks, carries dissolved silica. It penetrates cell cavities and begins depositing opal or micro-quartz gel.

3) Permineralization

Gel fills lumens (cell cavities), preserving vessels, tracheids, and rays like a cast. Early stages are often opal‑A/AG (amorphous silica).

4) Replacement and maturation

Over time silicon can replace cell walls and mature from opal into chalcedony/agate (microcrystalline quartz). The log becomes a hard stone, faithful to the original pattern.

5) Colors

Trace elements paint the palette: iron oxides (red/yellow), manganese (black), organic carbon (brown), copper/chrome (green, rarer). Open cavities may end with drusy quartz.

6) Erosion and exposure

Tectonics and erosion bring fossil forests to the surface. Polishing reveals growth rings and rays with gemstone clarity — a geology love letter to dendrology.

Palette

• From nutty to chocolate browns — a carbon/iron mix, classic “wood” appearance.
• Red and burgundy — hematite (Fe³⁺).
• Ochraceous/yellow — goethite/limonite (Fe³⁺ hydroxides).
• Carbon/black — manganese oxides or dense carbon films.
• Greens — trace Cu/Cr or chlorite; rare but desirable.
• White/gray — clean chalcedony/agate fills.

“Rainbow” petrified wood (famous in Arizona) shows multiple iron states and mineral phases in bold, adjacent areas.

Pattern words

• Growth rings — seasonal change of light/dark bands.
• Rays — radial bands (wood “tubes”) from pith to bark.
• Vessel pores — in hardwoods; ring-porous vs. diffuse-porous patterns.
• Knots and branch scars — swirling texture and figure.
• Agate veins — translucent silica bands “healing” cracks.
• Druse cavities — shiny “geodes” in former voids.

Photo tip: Side ~30° light brings out rays, and agate “windows” glow; a white reflector card on the opposite side enhances colors without glare.

Conifers (softwood)

Mostly tracheids (long, uniform cells), no vessels. Rays usually narrow. Some show resin canals. Growth rings often distinct: wide earlywood (spring), narrow latewood (summer) wood.

Deciduous (hardwoods)

Visible vessels/pores. Ring-porous species have large pores at early growth (oak/ash appearance); diffuse-porous distribute pores evenly (maple/poplar). Rays can be wide and prominent.

Palms and monocots

Not a true “wood”: look for scattered conductive fibers in parenchyma background — spotted patterns (“palm root”) instead of rays. Easily recognizable.

Color and inclusions

Hematite lines follow latewood; manganese stains rays dark; pale agate fills voids. Small druzy quartz sparkles may shine in cavities — microgeodes where sap flowed.

Species identification?

Possible to genus or family if well preserved, thin sections made, and comparative anatomy available. Many samples reasonably called “conifer” or “deciduous” without exact species.

Additional clue

Check the bark: preserved outer layer with lenticels (pores) — a rarity and helps determine orientation (bark → cambium → wood).

Peat bog wood / subfossil wood (bog oak)

Dark, water-saturated (hundreds to thousands of years), still organic. Light, smells like wood when cut, burns. Petrified wood — stone heavy, scratches glass.

Coal, jet, lignite

Resinous, softer than quartz; dark streak; often muted to semi-metallic luster. Petrified wood is hard, polished glassy and shows silica patterns.

Dyed wood or resin casts

Repeating patterns or neon solid color — a warning. Under a loupe natural pieces show cellular anatomy, not printed "striae".

Brecciated jasper

May resemble angular "wooden" patches but lacks rays/striae. 10× magnification — and wood anatomy wins.

"Palm root" vs. hardwood

Palms (monocots) feature spotted conductive fiber clusters without rays; hardwoods have pores + rays. Quick check with a loupe solves it.

Checklist

• Stone-hard; scratches glass (quartz hardness).
• Visible rays/striae/pores — like wood anatomy.
• Chalcedony/agate luster; possible druse cavities.

Classic locations

Arizona, USA — Triassic logs from the Chinle formation ("Rainbow" petrified wood). Yellowstone, USA — Eocene fossil forests buried in volcanic deposits. Washington State — Ginkgo petrified forest. Lesbos, Greece — Miocene ash-preserved forest.

World favorites

Madagascar — Triassic agatized wood with clear anatomy; Namibia — huge fossil trunks in the desert; Indonesia — abundant silicified wood for decorative slabs; New Zealand (Curio Bay), Argentina (Patagonia), etc. Fossil forests — surprisingly cosmopolitan.

Daily care

• Quartz hardness, but still prone to chipping from sharp impacts — don't try your luck.
• Large slabs are heavy: support evenly; lay felt on shelves.

Cleaning

• Lukewarm water + mild soap + soft brush; rinse and dry.
• Avoid harsh abrasives; silica is hard, but polishes can clog.
• Iron stains are sometimes removed by gentle, stone-safe chelators; test in an inconspicuous area.

Lapidary

• Orient cuts so that rings are visible on the face or rays — in quartered cuts.
• Beware of hidden cracks; stabilize if necessary before shaping the dome.
• Finish like agate: diamond abrasives → cerium oxide / other oxides; light pressure preserves bright anatomy.

Reading the rings

Use a loupe to follow the growth rings across the slab. Count them and look for narrow "stress years." You are traveling through the tree's biography.

Agate windows

Illuminate thin edges with backlight: agate veins and chalcedony halos glow, while denser zones remain opaque. It's like a forest with stained glass.