Wood Opal Identification Guide

What Wood Opal Looks Like

Wood opal is fossilized wood in which the original wood has been replaced by opal (hydrated silica, SiO2·nH2O). It preserves the woody structure — growth rings, grain, bark texture, and sometimes cellular detail — while the substance is now opal rather than chalcedony. Colors are typically earthy: tan, brown, gray, cream, yellow, and reddish, occasionally with a milky or even play-of-color (precious opal) sheen in rare specimens. Luster is waxy to vitreous or resinous, and the material is opaque to translucent.

Key Visual Cues

• Visible wood grain, growth rings, or bark texture
• Earthy browns, tans, grays; sometimes milky or fiery patches
• Waxy to resinous luster
• Log, branch, or limb shapes

Step-by-Step Field ID Checklist

1. Look for wood structure. Growth rings and grain are the giveaway that it is fossil wood.
2. Test hardness. Opal is softer (Mohs 5.5–6.5) — a steel point can scratch true wood opal, whereas agatized wood (Mohs 7) cannot.
3. Heft it. Opal is light for its size (SG ~1.9–2.2), lighter than chalcedony-replaced wood.
4. Check translucency. Often translucent on thin edges with a waxy glow.
5. Inspect for play-of-color. Rare precious wood opal flashes spectral colors.

Diagnostic Tests

• Mohs hardness: 5.5–6.5 — scratched by a hardened steel point (distinguishes it from harder agatized wood).
• Streak: White.
• Cleavage/fracture: No cleavage; conchoidal fracture.
• Density: ~1.9–2.2 g/cm³ — noticeably light.
• Acid: Inert to dilute HCl.

Common Look-Alikes and How to Tell Them Apart

• Agatized / chalcedony petrified wood: The most important distinction — chalcedony-replaced wood is harder (Mohs 7, will not be scratched by steel) and denser; wood opal is softer and lighter. Both preserve grain, so use a hardness/heft test.
• Common petrified wood (mixed silica): May combine opal and chalcedony; test multiple spots for hardness.
• Unaltered or charred wood / lignite: Much softer, lighter, and may smell or smear; not mineralized.
• Banded agate or jasper: Lacks woody grain structure entirely.
• Opalite (glass): Manufactured, uniform, with bubbles and no wood texture.

Where Wood Opal Is Found

Wood opal forms where silica-rich groundwater (often from volcanic ash) permeates buried wood and precipitates opal. Notable sources include Nevada (Virgin Valley produces precious opalized wood), other parts of the western United States, Indonesia (Java), and Australia. It is found in volcanic-ash-rich sedimentary beds and petrified forest deposits.

Collecting Tips

Search ash-rich sedimentary layers and petrified-wood localities for log and limb shapes showing grain. Distinguish opal from agatized wood by testing hardness with a steel point and by heft — wood opal is softer and lighter. Note that some wood opal (especially Virgin Valley material) can craze or crack as it dries, so stabilize fresh finds.