BOU / Formation & Geology · BOU005
Follow a source-visible model from deeply weathered sedimentary rocks and silica-bearing groundwater to Queensland ironstone concretions, seams and precious-opal relationships—without turning a teaching diagram into a specimen, mine or provenance claim.
Conceptual teaching model
The first two stages summarize a broad Australian sedimentary-opal model from Geoscience Australia. The later stages narrow to Queensland boulder-opal occurrence and gemological terminology. They are related, but not interchangeable.
Start with the broad Australian sedimentary-opal model: deeply weathered sedimentary rocks provide the setting in which silica can be mobilized and later deposited.
Scope · broad Australian model · BOU-SRC-001Evidence-led formation narrative
BOU does not collapse every source into one universal recipe. Each statement below names the level at which the source supports it.
Geoscience Australia describes deep weathering releasing silica into groundwater, with faults and joints acting as pathways and trapped silica-bearing water hardening into gel that can form opal veins and lenses.
Source · BOU-SRC-001
Queensland boulder opal can occur as veins or centre fillings in nodular ironstone accumulations chiefly composed of hematite and goethite.
Source · BOU-SRC-001
The Australian Museum distinguishes common opal from precious opal and uses “potch” as a miners’ term for grey common opal on Australian fields. Precious opal displays play of colour.
Source · BOU-SRC-011
Fields are not interchangeable
These observations come from GIA field reporting. They are examples of local geometry, not templates for every Queensland field or a way to assign provenance to a stone.
GIA documented ironstone concretions in soft sandstone and opal occurring along seams.
Sources · BOU-SRC-007
Open Koroit field recordGIA documented mostly siliceous ironstone concretions, other attached forms and multiple concretion-bearing beds.
Sources · BOU-SRC-007
Open Yowah field recordGIA documented opal-bearing ironstone concretions and field-specific centre/seam relationships.
Sources · BOU-SRC-007 · BOU-SRC-008
Open Quilpie field recordOpal + optics
The Australian Museum describes precious opal as an ordered three-dimensional grid of hydrated silica spheres that acts as a diffraction grating; common opal lacks the same ordered structure and does not show play of colour.
Sources · BOU-SRC-010 · BOU-SRC-011 · claim BOU005-CLM-005
What this model cannot tell you
These boundaries are part of the content, not fine print.
The provenance or exact field of an unknown specimen from appearance alone.
Whether an unknown stone is natural, treated, synthetic or authentic.
An appraisal, value, rarity or investment judgment.
A current mine boundary, tenure, access permission, road condition or safe travel status.
A literal specimen or mine cross-section from the conceptual diagram.
Source + claim ledger
Every BOU005 formation claim is mapped to existing BOU source IDs and rechecked on 2026-09-10. Source media remain reference-only unless asset-level reuse rights are separately verified.
Formation → specimen
Formation stages explain source-backed processes and field-scoped examples. They become specimen context only when a specimen record is already linked to a locality by documentation.
Failure mode: matching ironstone, seam geometry or play-of-colour to a field is not a documented origin claim.
ICU / CONNECTED DISCOVERY
These are bounded editorial connections; paid placement does not influence them.
Loading public connections…