Opalised Wood

How it forms

Opalised wood is fossil material produced when wood is infilled or replaced by opal; it results from petrification—permineralization plus replacement. In the NPS account, permineralization means minerals filling the natural pores of original organic material; it occurs when groundwater carrying silica moves through minute pores in buried wood (or bone or shell), often preserving a three-dimensional internal mould at cellular or microscopic level. Silicification—infilling and/or replacement by silica—is the commonest kind, particularly in fossil wood and bone, and the silica may take several forms, among them 'Opal: hydrated amorphous silica'. The silica source is chiefly the weathering of volcanic ash (volcanic glass), whose silica is not locked inside a mineral lattice and is therefore readily transported by groundwater to precipitate in or replace woody material. Where the silica that infills or replaces wood tissue is opal, opalised wood is the result. Its parent-material difference from ordinary opal is that ordinary opal forms by silica gel solidifying in rock cracks or layers, whereas opalised wood preserves the form of wood tissue (even cell structure) while opal fills or replaces plant organic tissue. Plant-origin markers such as rootlet fossils are also seen in Ethiopian opal.

Scope: Used to describe fossil material formed when woody tissue is infilled or replaced by opal; the parent body is plant organic tissue, distinguishing it from ordinary opal, in which silica gel solidifies directly in rock cavities.

Where it forms

Groundwater system in a burial environment: silica-bearing groundwater passing through the tiny pores of buried wood is the medium of permineralization/replacement.

Volcanic-ash-rich (volcanic-glass) settings, or volcanic mudflow (lahar) settings that rapidly bury forests: volcanic ash is a ready silica source and rapid burial favors preservation.

How it forms, step by step

1. Silica-bearing groundwater flows through the tiny pores of buried wood (or bone or shell).

   Sources NPS Permineralization

   • Permineralization and Replacement

2. Silica infills the wood's natural pores by permineralization, often preserving a microscopic/cellular three-dimensional internal mold, or replaces the woody tissue.

   Sources NPS Permineralization

   • Permineralization and Replacement

3. The silica comes mainly from weathering of volcanic ash (volcanic glass), whose silica is readily carried by groundwater.

   Sources NPS Permineralization · NPS Volcanoes and Fossils

   • Permineralization and Replacement
   • Volcanoes and Fossils

4. When the silica infilling/replacing the wood tissue takes the form of opal (hydrated amorphous silica), opalised wood is produced.

   Sources NPS Permineralization

   • Permineralization and Replacement

Link to appearance

Preserves the external form of wood (potentially including growth rings and cell structure) while its mineralogy is replaced or infilled by silica (opal); it may be play-of-color precious opal or common opal.

Timescale

The cited sources give no numeric age for the petrification/replacement of opalised wood; they describe the process as occurring after burial and during diagenesis.

Related base materials

• Opal (hydrated amorphous silica): replacement-of | Opal fills the cell pores of the wood or replaces the woody tissue
• Wood (organic tissue): precursor-of | Wood is the precursor material, petrified by silica (opal) through permineralisation/replacement

Supporting findings

Recorded localities

Authoritative sources list the localities below; the list is not exhaustive.

Europe

• Ihráčska Píla, Žiar nad Hronom District, SlovakiaMineral specimen locality (not a source of jewellery material)
  Sources Mindat: Opal

  • Opal: Mineral information, data and localities
• Rohožnica, Veľký Ďur, SlovakiaMineral specimen locality (not a source of jewellery material)
  Sources Mindat: Opal

  • Opal: Mineral information, data and localities

These are localities recorded by authoritative sources, not an origin finding for any individual piece. The CIBJO Gemstone Book (2022, §4.2.3) says a place of origin names where a gem was mined or harvested, is a matter of opinion, does not imply a level of quality, and that cutting, processing or exporting centres must not be used to imply origin. See the origin determination guide and the overview of recorded localities.

Causes of colour and optical effects

How it is identified

Other research findings

Limits of this research

• Research status: Formation, occurrence or other material-specific findings found in authoritative sources
• The NPS text describes fossil silicification/permineralization in general and does not specifically define the name 'opalised wood'.
• The evidence for 'infilled/replaced by opal' is that opal is among the silica forms involved; the source does not quantify how often opal is the replacing phase.
• The plant-origin markers such as rootlet fossils come from descriptions of Ethiopian opal inclusions, not from research specific to opalised wood.
• The cited sources give no specific numeric age.

Methodology

• Origin determination and its limits
• Quality grading and value factors
• How gemmology became a technical discipline

Evidence grades

A

Standards bodies, government geological surveys, peer-reviewed research and museum records

B

Institutional educational material

C

Specialist databases or cautious readings

A statement is never graded above its strongest source.

This section is LumaLark's own factual restatement of public authoritative sources, credited item by item; it contains no verbatim excerpts, images or third-party logos.